New integral properties for Potential Vorticity and applications to the

ocean dynamics

MOREL Y.1, GULA J.2, PONTE A.2

1 CNRS/LEGOS, France

2 LOPS, France

Since the work of Ertel (1942), Potential Vorticity (PV) is a recognized as a key quantity to study

the circulation and in particular to understand geophysical turbulence. Many processes can indeed be

explained by combining PV conservation properties and the PV inversion principle. Diabatic processes

transform PV but this transformation is subject to constraints that allows to use "PV thinking" even when

there exists mixing.

As a lagrangian quantity, PV evolution is more naturally examined along isopycnic/isentropic

surfaces (for conservation or inversion), which unfortunately complicates its calculation : diagnosing PV

along isopycnic surface often requires numerical interpolations whose associated errors can be too strong to

obtain a proper view of the PV evolution. Most validation of realistic models using PV thus remains

qualitative. Similarly, PV is also not easily measured in situ.

We propose new diagnostics and integral constraints, based on an original expression of PV,

which we believe allows easier calculation and interpretation of PV evolution. We discuss the application

of these results to :

- the role of turbulence on the general circulation

- the generation of PV at the bottom boundary layer

- the link between surface density anomaly and interior PV anomalies for isolated vortices

Oral preference

1 CNRS/LEGOS, France

2 LOPS, France

The generation of submesoscale eddies and of turbulence by a row

of mesoscale surface eddies in the Sea of Oman MORVAN M.1, CARTON X.1, L'HEGARET P.2

1 LOPS/IUEM, UBO, Brest, France 2 MPO/RSMAS, Miami, USA

The Persian Gulf is a source of high salinity water which flows into the Oman Sea via the

Strait of Hormuz. There it mixes with surrounding Indian Ocean Surface Water by cascading down

the continental slope. After the strait, this salty plume stabilises as a density current along the Omani

Slope (in the Sea of Oman). Then the mesoscale eddies spreads the Persian Gulf Water (PGW) from

this slope current, offshore.

During the spring 2011 and 2014, the Phys-Indien 2011 [L’Hégaret et al., 2016] and

Phys-Indien 2014 surveys revealed the presence of submesoscale eddies at the depth of the PGW in

the Oman Sea. In the same periods of time, the altimetry suggests that the surface dynamics was

dominated by a row of mesoscale eddies alternatively signed.

To investigate the origin and the dynamics of such submesoscale eddies, we set up

idealized simulations of a row of mesoscale surface intensified eddies alternatively signed, in a

channel, by using the primitive equation ROMS model [Shchepetkin and McWilliams, 2005].

These experiments show that the submesoscale eddies are essentially generated by

instabilities and friction in the first hundred meter depth. Once generated, they are advected offshore

by the mesoscale eddies, then warped by the surrounding shear. As a result, the PGW is carried

offshore by the submesoscale eddies and subsequently spread by the surrounding shear.

Complementary diagnostics are run to conclude about the production rate and lifetime of such

submesoscale eddies.

Also, below the mesoscale surface intensified eddies down to the bottom, a turbulent

wake-like dynamics is generated over a longer period of time. We note that the generated

submesoscale eddies are smaller at depth than near the surface. Kinetic and potential energy spectra,

and potential enstrophy spectra are calculated to determine the respective direct and reverse

cascades, and the role played by instabilities, shear or vortex merger in these cascades is studied.

Complementary diagnostics are run to isolate the mechanism triggering this generation of

submesoscale eddies at depth. In particular, vortex stretching and topographic generation are

examined.

Poster preference

Interactions between balanced and imbalanced modes

NADIGA B.1, CASPER W.2

1 LANL 2 University of Washington

We report on linear stability and (nonlinear) numerical experiments conducted using the nonhydrostatic

Boussinesq equations and the shallow water equations with the aim of probing interactions between balanced

and imbalanced modes of the respective systems. In a previous study using the nonhydrostatic Boussinesq

equations (Nadiga BT. Nonlinear evolution of a baroclinic wave and imbalanced dissipation. Journal of Fluid

Mechanics. 2014 Oct 10;756:965-1006.) it was found that the the dissipation of balanced energy by imbalanced

processes scales exponentially with Rossby number of the base flow. In addition to other analyses, the

counterpart of such a scaling in the shallow water system is considered.

No preference

Internal gravity waves as mediator of mixing and drag in the ocean

circulation

OLBERS D.

Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Internal gravity waves play a major role in the energetics of oceanic circulation. Powered by energy

supply from mainly tides and wind they interact with the mean circulation and eddy field, ultimately transferring

energy to small-scale turbulence by wave breaking, thus mediating the energy cascade from large-scale to small-

scale motions and the feed-back via density mixing. Global climate models usually resolve at most a small part

of the full spectrum of gravity waves so that mixing induced by waves and wave-mean-flow interaction need

special parameterizations. An energetically consistent model of the mixing and wave drag requires a closed

model of the wave energetics, including generation, wave-mean-flow interaction, non-linear transfer within the

wave field and dissipation. The IDEMIX (Internal wave Dissipation, Energetics and Mixing) framework meets

these requirements by heavy truncation of the radiation balance equation: the energetics is formulated for the

evolution of the wave field in phase space and reduced to a small number of compartments by integrals over

respective parts of the spectral wavenumber space. The model is working in physical space - the global ocean -

with wave propagation by spectrally averaged group velocities. The IDEMIX concept yields an energetically

consistent and practical framework to describe wave effects and has been shown to be successful for ocean

applications: it predicts plausible magnitudes and three-dimensional structures of internal wave energy,

dissipation rates and diapycnal diffusivities in rough agreement to observational estimates. We briefly review the

most important sources, sinks and interaction processes in which the gravity wave field in ocean is involved,

placing emphasis on parameterizations suitable for implementation into a spectrally oriented radiative transfer

approach. Wave breaking processes, critical layer effects and wave refraction in a sheared mean flow are

specifically addressed in this discussion. The construction of IDEMIX models is illustrated.

Oral preference

Direct measurements of mixing efficiency from ocean mixing glider

measurements

PALMER M.1, PELLING H.1, RIPPETH T.2, MONISMITH S.3, TOBERMAN M.4, INALL M.4

1 NOC, UK 2 Bangor University, UK

3 Stanford University, UK 4 SAMS, UK

Recent decades have seen significant advances in our capability to directly measure turbulence

using microstructure probes and acoustic techniques. Yet, there remains considerable debate over the

mixing efficiency (Γ) across the broad range of ocean stratification. It is intuitive that within an already

well-mixed fluid, any level of additional energy will not act to increase mixing, and so has an efficiency

approaching zero. Over the spectral range of stratification, and under varying contributions from a variety

of candidate mixing mechanisms it is also intuitive that the mixing efficiency will be highly variable,

dependent on the intensity of the turbulent event. Despite this, there is plenty of literature available that

appears to confirm that, on average, the turbulent kinetic energy dissipation rate (ε) has a near constant

value of Γ ~0.2, as suggested by Osborn (1982). There are however, few investigations that directly

compare ε with the immediate effects of mixing over any substantial timescale to enable analysis of Γ

under variable forcing.

We present 20 days of measurements from the Ocean Microstructure Glider (OMG) to examine

coincident measurements of ε and thermal variance (χ) to derive Γ over a range of forcing scenarios. The

OMG provides profiles of ε and χ every 15-20 minutes from the near surface to 100 m, covering the

seasonal thermocline which is situated at approximately 50 m depth. We examine the apparent change Γ,

relative to turbulent intensity, represented by the buoyancy Reynolds number (Reb). Within strongly-

stratified, low-intensity regions, typified by low Reb, we confirm Γ ~0.2. With increasing intensity

however, Γ reduces near linearly becoming several orders of magnitude less efficient in weakly stratified,

energetic regions. We compare our results with those from observational data collected using similar

methods and to proposed solutions for Γ from LES model results, such as those presented by Shih et al,

2005.

Oral preference

Shear-driven mixing at high buoyancy Reynolds numbers

PASSAGGIA P.-Y., WHITE B., SCOTTI A.

University of North Carolina

Turbulent mixing is a critical component of the ocean energy budget [Wunsch & Ferrari, 2004],

contributes to the closure of the Meridional Overturning Circulation [Talley, 2013], and is an important driver of

coastal processes [Moum et al., 2013]. However, despite decades of research, we still lack a precise

understanding of how mixing in a stratified environment depends on the environmental conditions that drive it

[Ivey et al., 2008].

The study of mixing in stratified flows has received a lot of attention over the last 40 years, several

studies have been devoted to stratified turbulence generated by a grid towed or oscillated in a linearly stratified

fluid. However, since the early work of Britter (1974), there have been only few experiments that focused on the

turbulent dynamics in the presence of a vertical shear [Odier et al., 2014].

We propose a new experiment that allows for performing shear-driven stratified experiments at high

buoyancy Reynolds numbers while keeping the Richardson number constant and close to 1/4. For these

conditions, mixing is driven by Kelvin-Helmholtz instabilities for Reynolds numbers close to 105, representative

of the naturally occurring shear instabilities observed for instance by Geyer et al. (2010) or Seim & Gregg (1994)

on field sites.

The present setup consists of a 36 m (long) x 1.2 m (high) x 0.75 m (wide) stratified recirculating tank

where a shear profile is created by injecting dense salty water through a slit. The jet coming out from the slit

forces a linear type velocity profile that mixes a linearly stratified density profile, triggering instabilities,

turbulence and mixing.

Using combined measurements of particle image velocimetry, laser induced Fluorescence, Acoustic

Doppler velocimetry, hot wire anemometry and conductivity-temperature-depth measurement, we show that

mixing is essentially driven by Kelvin-Helmholtz instabilities and that secondary instabilities, located in the

braids play a major role in the overall process [Geyer et al., 2010].

Buoyancy and momentum fluxes are used to estimate the flux Richardson number, which define the

mixing efficiency and confirm the prediction that even for high Buoyancy Reynolds numbers, the mixing

efficiency for shear driven turbulence remains of the order of 0.1, consistent with the parametrization for the

general circulation models of Olbers & Eden (2013), Polzin (2009) and St Laurent et al. (2002).

Oral preference

Influence of mixing on tracer evolution in stratified flows: theoretical

aspects and numerical results.

PENNEY J.1, MOREL Y.1, HAYNES P.2, AUCLAIR F.3, NGUYEN C.3

1 CNRS/LEGOS, France

2 DAMTP, France

3 Toulouse University, France

We study the fundamental physics of coupling between dynamics and (possibly active) tracers in

stratified flows. Idealised configurations (2D and 3D), using non-hydrostatic models, are developed to

investigate the effect of small scale turbulence and mixing on the distribution of tracers.

Turbulence is here associated with the development of Kelvin-Helmholtz shear instabilities.

We focus on the spatial distribution and evolution of the local concentration of tracers during the

different instability and mixing phases. During the development of instabilities, tracers are stirred as a

function of the local properties of the –turbulent- velocity fields. The turbulent cascade eventually leads to

mixing and homogenization of tracers at small scales with a final distribution that then remains fixed along

density surfaces, and which prescribes the further localisation and transport of the tracer.

The final profiles of density and tracers depend on the local volumetric ratio of tracers in small

regions of the flow during the instability phase. In this perspective, any preconditioning process acting on

the local distribution of tracers and density prior to the homogenization phase may have an impact on the

final redistribution of tracers. As the ratio of tracers can greatly vary within the Kelvin-Helmholtz billows,

a hyper-sensitivity to details of the turbulence preconditioning can be expected for the final profiles.

However, theoretical and numerical results will be presented showing that tracers cannot evolve

freely with respect to density and that the redistribution of tracers within density ranges is constrained.

Differential diffusion of tracers is also shown to potentially lead to stronger diapycnal mixing of tracers.

Oral preference

1 CNRS/LEGOS, France

2 DAMTP, France

3 Toulouse University, France

Direct numerical simulation of Rayleigh-Taylor instabilities subject to

double-diffusion

PENNEY J.1, STASTNA M.2

1 LEGOS, France 2 University of Waterloo , Canada

Double-diffusive effects arise when the density of a fluid depends on two components with different

diffusivity coefficients. The more rapid diffusion of one component (e.g., temperature) relative to the other (e.g.,

salinity) can cause local density anomalies resulting in instabilities. In a two tracer system, if the driving

instability is due to an unstable density stratification, double-diffusion may still affect the overall flow

development. This work presents the direct numerical simulation of a three-layer system comprised of upper and

lower layers of denser, cool, fresh water, and an intermediate layer of lighter, hot, salty water. The unstable

upper pycnocline results in the development of Rayleigh-Taylor (RT) instabilities. As these instabilities grow,

and reach their finite amplitude manifestations, the rapid diffusion of the temperature field can create regions of

density outside of the range of the initial conditions, resulting in a greater density difference between the sinking

RT instabilities and the surrounding fluid. The resulting buoyancy drives motions that are more energetic than

the mature stage of single constituent RT instability, and greater variability of the salinity and density fields

compared to the temperature and density fields. Through two-dimensional spectral analysis, it is confirmed that

the salinity and density fields have a stronger contribution from smaller spatial scales than temperature and

kinetic energy fields.

No preference

How does internal tide generation vary in the horizontal?

POLLMANN F.1, NYCANDER J.2, EDEN C.1, OLBERS D.1

1 Hamburg University, Germany 2 Stockholm University, Norway

Breaking internal gravity waves are considered to be a major source of small-scale turbulence, which

acts to mix density in the vertical and thus contributes to driving the global overturning circulation.

One major forcing mechanism of internal waves is the energy transfer from the barotropic tide when it

interacts with rough topography. The pioneering work by Bell (1975a,b) to analytically compute this energy

transfer has been extended in the past decades, but the directional dependence of the flux has so far not been

considered. This piece of information, however, is crucial for a correct setup of internal gravity wave models and

mixing parameterizations which are based on internal wave energetics.

We here propose an analytical method to describe the horizontal variations of the far-field energy flux.

Since its main assumption of a bounded source region is not met in reality, we subdivide the ocean bottom into

overlapping circular patches. The energy flux density is computed separately for each patch, neglecting the

influence of the topography outside of the particular patch. We first evaluate this approach using idealized

topography before calculating the angle-dependent energy conversion for the global ocean. The long-term goal is

to add this information to IDEMIX, a mixing module that predicts turbulent kinetic energy dissipation rates

based on internal gravity wave energy, in order to improve numerical simulations of ocean dynamics.

Oral preference

Pycnocline Mixing is Seasonally Stratified Shelf Seas

POLTON J., GUIHOU K., BRERETON A., LUNEVA M.

National Oceanography Centre

The shelf seas, relative to their size, contribute a disproportionate fraction to total global primary

production. In seasonally stratified shelf seas mixing processes at the pycnocline mediate the transfer of

biological and physical fluxes that in turn control this biogeochemical cycle. Regional models generally

poorly simulate mixing processes at the pycnocline, instead forming an interface between surface driven

mixing and bottom boundary layer driven mixing. It is hypothesised that internal waves, missing from shelf

wide regional models, are responsible for missing mixing. Internal tides are diagnosed in a 1.8km

Northwest European shelf NEMO simulation and favourably compared against FASTNEt hydrographic

observations. We anticipate this new class of fine resolution simulation, which can simulate non-local

propagation of momentum along the pycnocline, forms the basis of improved parameterisations for shelf

sea mixing. Large eddy simulation techniques are developed to explore this transition to mixing that must

otherwise be parameterized in regional models.

Oral preference

Fate of internal solitary waves in Manado Bay, Indonesia

ADI PURWANDANA1,2

, YANNIS CUYPERS1 , PASCALE BOURUET-AUBERTOT

1 AND DANIEL BOURGAULT

3

1Laboratoire d'Océanographie et de Climatologie par Experimentation et Approche Numérique

(LOCEAN), Université Pierre et Marie Curie (UPMC) 2Research Center for Oceanography, Indonesian Institute of Sciences (RCO-LIPI)

3Institut des Sciences de la Mer de Rimouski, Université du Québec à Rimouski, Canada

Indonesian seas and it surrounding seas have been identified as the most active region in

generating internal solitary wave (ISW). The ISW propagation in this region even can be clearly

identified from MODIS satellite images [Jackson, 2007]. Strong internal tides in these regions can

evolve into trains of ISW due to large number of shallow interisland sills that separate deeper basins

[Nagai et. al. 2014]. Near shore breaking of these ISWs can drive strong water mass transformation.

We present an analysis of shoaling ISW in Manado Bay, Sulawesi Island. This study is the first

observational study of shoaling internal waves in Indonesian Seas. The observation was carried out

in May 2009 in Manado Bay waters using Research Vessel Baruna Jaya VIII RCO-LIPI anchored in

a fixed position around 2 km from the coast of Manado Bay. Shipboard ADCP 75 kHz RD

Instrument and single beam echo sounder 120 kHz Simrad EK500 were operated during the

experiment to measure the current profiles and scattering layer, respectively. Watermass properties

were measured with CTD SBE Seacat Profiler. Repeated CTD profiling (yoyo) were conducted

during sequences of high frequency waves recorded by the echosounder. Both a large wave of

depression and sequence high frequency waves of elevations are observed. We use a simple

Korteweg-deVries (K-dV) model to fit the large wave of depression whose amplitude reaches 40 m.

The K-dV fit implies an available potential energy of 39.59 MJ/m. The sequence of high frequency

waves of elevation did not show any consistent fit with the Kdv equation and could be rather

interpreted as boluses running up the slope. The waves also exhibit strong turbulence, Thorpe scale

analysis of the density inversions, reveals Thorpe scale up to 20 m; high eddy kinetic energy

dissipation up to 10-4

m2s

-2 and diffusivity up to 10

-3 m

2s

-1. To better understand their dynamics, a

simulation based on a 2D non-hydrostatics model [Bourgault and Kelley, 2004] is also carried out to

examine the behaviour of shoaling of ISWs propagating in a background environment representative

of the conditions prevailing in the Sulawesi Sea. The shoaling ISW of depression with initial

amplitude of 25 propagating from Sulawesi Sea compares well with the observations, depicting some

boluses run up in the bottom layer bringing tongue of denser water masses as observed in the

echosounder and measured density profiles.

REFERENCES

Bourgault, D., Kelley, D. E., 2004. A laterally averaged nonhydrostatic ocean model. J. Atmos. Oceanic

Technol. 21, 12, 1910–1924.

Jackson, C., 2007, Internal wave detection using the Moderate Resolution Imaging Spectroradiometer

Nagai, T., and T. Hibiya, 2015, Internal tides and associated vertical mixing in the Indonesian Archipelago, J.

Geophys. Res. Oceans, 120, 3373–3390.

1 Tour 45-46, 5e étage 4 Place Jussieu 75252 Paris Cedex 05, France 2 Jl. Pasir Putih I Ancol Timur, 14430, Jakarta, Indonesia 3 Allée des Ursulines, Rimouski, QC, G5L 3A1, Canada

Preliminary results over the Med-CORDEX domain of a new high

resolution Regional earth system model with an active

biogeochemical component

REALE M.1, 2, SOLIDORO C.2, GIORGI F.1, DI BIAGIO V.2, MARIOTTI L.2, FARNETI R.1

1 ICTP, Trieste,Italy 2 OGS, Trieste,Italy

Here we illustrate some preliminary results with the RegCM-ES (Earth System

Regional Climate Model) model, developed and used at ICTP [Sitz et al., 2017], with an active

biogeochemical component (BFM, Biogeochemical Flux Model, Vichi et al., 2015). The new

coupled model has an atmospheric component (RegCM 4.5, Giorgi et al., 2015) with a spatial

resolution of 20 km , an ocean component (MITgcm, Marshall et al., 1997) with a resolution of

1/12 degree and a component to simulate river discharge (HD, Hagemann et al., 2001).

RegCM-ES has been implemented and tested over the Med-CORDEX domain (Mediterranean

region, Giorgi et al., 2009). Preliminary comparisons with climatological data and available

reanalysis show that the new coupled model is able to reproduce the spatial pattern and the

seasonal variability of phytoplankton and zooplankton as well as of nutrients as phosphate and

nitrate in the domain of study and, in particular, in areas where vertical mixing is substantial,

e.g. in the gulf of Lions and in the southern Adriatic.

REFERENCES:

Giorgi, F., Jones, C., and Asrar, G.: Addressing climate information needs at the regional level: the CORDEX

framework, World Meteorol Organ Bull, 58, 175-183, 2009

Giorgi, F., Coppola, E., Solmon, F., et al.: RegCM4: model description and preliminary tests over multiple

CORDEX domains, Clim. Res, 52, 7-29, 2012

Hagemann, S. and Dmenil, L.: Validation of the hydrological cycle of ECMWF and NCEP reanalyses using the

MPI hydrological discharge model, J. Geophys. Res., 106, 1503-1510, 2001

Marshall, J., Adcroft, A., Hill, C., Perelman, L., and Heisey, C.: A finite-volume, incompressible Navier Stokes

model for studies of the ocean on parallel computers, J. Geophys. Res., 102, 5753-5766, 1997

Sitz, L., F. Di Sante, R. Farneti, R. Fuentes-Franco, E. Coppola, L. Mariotti, M. Reale,G. Sannino, M. Barreiro,

R. Nogherotto, G. Giuliani, G. Graffino, C. Solidoro, G. Cossarini,and F. Giorgi : Description and evaluation

of the Earth System Regional ClimateModel (RegCM-ES) (2017,in submission)

Vichi, M., Cossarini, G., Gutierrez Mlot, E., Lazzari, P., Lovato, T., Mattia, G., Masina, S., McKiver, W.,

Pinardi, N., Solidoro, C., and Zavatarelli, M.: The Biogeochemical Flux Model (BFM): Equation

Description and User Manual. BFM version 5.1, Tech. rep., BFMReport series N. 1, Bologna, Italy, 89 pp,

2015.

Poster preference

An Energetically Constrained Ocean Surface Boundary Layer

Parameterization including Surface Wave Effects for Climate

Applications

REICHL B.1,2, HALLBERG R.1, GRIFFIES S.1, ADCROFT A.2, LI Q.3, FOX-KEMPER B.3

1 NOAA GFDL, 2 Princeton University,

3 Brown University

Turbulent vertical mixing plays a critical role in determining the conditions of the

upper-ocean and feedbacks to the air-sea fluxes. For this reason, accurate parameterization of

this mixing is an important component for simulating coupled processes ranging from tropical

cyclones to the Earth’s climate system. Popular methods for modeling the upper ocean turbulent

mixing for climate applications often suffer from either poorly constrained energetics or

burdensome computational requirements. The bulk (integrated) boundary layer approach

reduces the computational requirement by vertically integrating the boundary layer, but the loss

of vertical information is undesirable for many applications. To overcome these obstacles, a

new model has been developed which utilizes ideas from the bulk boundary layer while

maintaining finite, vertical eddy mixing coefficients. The new model solves an integrated,

equilibrium turbulent kinetic energy (TKE) equation including parameterized TKE sources and

dissipation. Shear, buoyancy, and surface wave (Langmuir and breaking) driven mixing are

parameterized based on more complete, high-resolution (but computationally demanding)

simulations. These high-resolution results include several Large Eddy Simulations, which are

supplemented with a comprehensive set of one-dimensional model runs (using k-epsilon two-

equation turbulence closure) to cover a wide range of model forcing and initialization

conditions. Results show that the new parameterization is able to produce similar profiles of

turbulent mixing compared to the more demanding k-epsilon parameterization that solves

additional prognostic equations. This parameterization is thus proposed as a robust, cheap

method suitable for simulating the ocean surface boundary layer on scales appropriate for

climate implementation. This presentation will particularly emphasize the development of the

Langmuir turbulence parameterization, including its implementation in coupled climate

simulations.

Oral preference

Do observations adequately resolve the natural variability of

oceanic turbulence: Revisited.

RIPPETH, T.P.1 AND MOUM, J.N.2

1Bangor University, School of Ocean Sciences, United Kingdom

2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, USA

Following the 2007 Liege Ocean Turbulence conference we presented a paper entitled

“Do observations adequately resolve the natural variability of oceanic turbulence?” in which we

concluded that “turbulence measurements when made in a comprehensive and systematic manner …

are providing significant insights into ocean dynamics” but that “the larger oceanographic problem of

defining the full geographic variability of mixing remains”. In this contribution we will present a

number of new observations, made using a range of methods, including new moored instrumentation

(eg. Lucas et al, 2014; Moum and Nash, 2009) which have greatly expanded our sampling capacity

(to a seasonal cycle and more). Observational insights include a time series showing a 3-order of

magnitude variability in open ocean dissipation (and by implication mixing) in a tidal cycle, over

steep topography (eg. Mead Silvester et al, 2014). They also show a rapid decline in dissipation

moving away from steep topography (eg. Inall et al, 2005). Whilst these, and other new observations,

further highlight both the need for, and the problem of, defining the full geographic variability of

mixing, they also provide use with insights as to how we can refine measurement campaigns, using

these newly development measurement techniques, to better capture the variability in dissipation and

hence mixing.

REFERENCES

Inall, M., Rippeth, T., Griffiths, C. and Wiles, P., 2005. Evolution and distribution of TKE production and

dissipation within stratified flow over topography. Geophys. Res. Letts., 32, L08607

Lucas, N.S., Simpson, J.H., Rippeth, T.P. and Old, C.P., 2014. Measuring turbulent dissipation using a tethered

ADCP. J. of Atmos. and Oceanic Tech., 31, 1826-1837.

Mead Silvester, J., Lenn, Y-D, Polton, J.A., Rippeth, T.P., and Morales Maqueda, M. 2014. Observations of a

diapycnal shortcut to adiabatic upwelling of Antarctic Circumpolar Deep Water, Geophys. Res. Lett., 41,

7950-7956.

Moum, J.N. and Nash, J.D. 2009: Mixing measurements on an equatorial ocean mooring, J. Atmos. and

Oceanic Tech., 26, 317-336.

Moum, J.N. and Rippeth, T.P, 2007. Do observations adequately resolve the natural variability of oceanic

turbulence? J. Mar Sys, 77, 409-417.

You can send your abstract online or by e-mail to oceanphys@ulg.ac.b

Multiscale analysis of ocean color turbulent heterogeneities:

comparisons of SST and Chl-a multifractal properties using 2D

structure functions

RENOSH P.R.1, SCHMITT F.G.2, LOISEL H.3

1 Conservatoire National des Arts et Métiers, Laboratoire CEDRIC, France 2 CNRS, Laboratory of Oceanology and Geosciences, France

3 Université du Littoral Côte d’Opale, Laboratory of Oceanology and Geosciences, France

Satellite remote sensing is a powerful tool for understanding many of oceanic processes

synoptically. Renosh et al. [2015] proposed a methodology for analyzing the multiscaling properties

of satellite images using tools borrowed from the field of turbulence and multifractal analysis. The

present work addresses the practical application of this study into different oceanic regions.

For this purpose we have selected satellite ocean color images of Remote sensing

reflectance (Rrs), Chlorophyll-a (Chl-a) and thermal infra-red Sea surface temperature (SST). For

understanding the spatial scaling associated with turbulence, it is important to have a daily imagery of

these products. As far as ocean color remote sensing is considered, it is very difficult to have cloud

free images for the understanding of scaling behavior. We have identified seven contrasted regions of

the global ocean, characterized by high spatial heterogeneity in Chl-a and SST. Power spectral

analysis has first been used for the present study. Here we use 1D and 2D Fourier power spectra to

understand the spatial scaling of Chl-a and SST. The 2D spectral slope β is derived from the 2D

power spectrum using radial sum of the power spectrum.

The multi-scaling properties of these images are also studied using the 2D Structure

Function (SF) method. Using a lognormal fit, we have derived the multifractal parameters (Hurst

exponent H and intermittency parameter µ) of these images using SF method. The derived

multifractal parameters show variability in their values depending upon the region. The scatter plot of

µ versus H shows some clustering of these parameters. The SST is showing low intermittency, Rrs

high, and Chl-a shows intermittency in between SST and Rrs for all the regions, indicating biological

activities, which are weaker in oligotrophic regions.

This approach can be applied even for irregular images with missing data, and help to

characterize the heterogeneities, and the scale dependence and intensity of physics-biology coupling

at sub-mescales using ocean color satellite images.

REFERENCES

Renosh, P. R., Schmitt F. G., and Loisel H., 2015. Scaling analysis of ocean surface turbulent heterogeneities

from satellite remote sensing: use of 2D structure functions. PloS ONE10(5), e0126975.

Turbulence and mixing in the NW Iberian shelf in response to upwelling

events

RUIZ VILLARREAL M.1, GARCÍA-GARCÍA L.M.2, MARTA-ALMEIDA M.3, MOURIÑO-CARBALLIDO B.4, COBAS M.5

1 Instituto Español de Oceanografía (IEO), Spain 2 CEFAS, UK

3 Campus do Mar IEO-Uvigo, Spain 4 University of Vigo, Spain

5 University of East Anglia, UK

The NW Iberian shelf is influenced by upwelling events that prevail in spring-summer and strongly

shape a highly productive ecosystem. Renewing of nutrients is driven by the upwelling of bottom waters, and the

response of the shelf system to upwelling events is complex. In this presentation, we will study by means of a 3d

model configuration of the ROMS model different upwelling events where concurrent multidisciplinary

sampling of plankton with semi-automatic methods and hydrography (including dissipation rate of TKE) was

carried out in order to study the response of the plankton community to the phases of upwelling winds. The

impact of the variability of circulation and mixing on the plankton community will be evaluated. Special

reference will be given to the sensitivity of the coupled 3d hydrodynamical-ecological model configuration to the

turbulence closure.

Oral preference

A Modification to the Structure Function Method to Correct for the

Impact of Wave Orbital Velocity Shear

SCANNELL B, SIMPSON J. H. AND RIPPETH T. P.

Bangor University, United Kingdom1

The structure-function methodology is an established technique for calculating the

turbulent kinetic energy (TKE) dissipation rate, ε, from velocity profiles such as those obtained with

an acoustic Doppler current profiler (ADCP) [Wiles et al., 2006; Lucas et al., 2014]. Turbulent

velocity profiles are calculated by removing the background shear averaged over multiple

measurement profiles during an observation period. TKE dissipation is then calculated as a function

of the growth in the variance of the along-beam turbulent velocity with separation distance through

relationships derived by Kolmogorov.

However, instantaneous beam coordinate velocity profiles for a vertically-oriented ADCP

have the potential to be contaminated by any fluctuating velocity shear due to wave orbital motion.

Since the wave period is much shorter than the averaging period normally used to determine the

statistical properties of the turbulent motions, the standard procedure for calculating the structure

function does not separate the turbulent velocity variation from the wave orbital velocity shear signal,

potentially resulting in biased estimates of TKE dissipation rate.

The theoretical wave orbital velocity shear varies linearly with the separation distance, r,

over which the shear is measured, hence it’s contribution to the structure function varies as rn, where

n is the order of the structure function. By contrast, the turbulent velocity structure function varies as

a function of rn/3. Here we modify the structure function methodology on this basis to provide a

simple, physically-based means of separating the true turbulent component of the structure function

from the non-turbulent contribution arising due to wave orbital velocity shear.

The revised approach is then validated against observations from a long-term mooring in a

tidally swept, seasonally stratified shelf sea. The mooring was equipped with three high frequency

pulse-pulse coherent ADCP, making observations centred at depths of circa 24, 42 and 52m in an

area with a typical water depth of 150m. Applying the modified method to observations during

winter 2014/15 when the water column was unstratified produces results that show a significant

improvement in agreement with standard depth scaling under a variety of wave conditions and

contrasting surface forcing dominated by both (1) convective buoyancy flux and (2) significant wind

stress.

REFERENCES

Lucas, N. S., J. H. Simpson, T. P. Rippeth, and C. P. Old (2014), Measuring Turbulent Dissipation Using a

Tethered ADCP, J. Atmos. Ocean. Technol., 31(8), 1826–1837, doi:10.1175/JTECH-D-13-00198.1.

Wiles, P. J., T. P. Rippeth, J. H. Simpson, and P. J. Hendricks (2006), A novel technique for measuring the rate

of turbulent dissipation in the marine environment, Geophys. Res. Lett., 33(21), 1–5,

doi:10.1029/2006GL027050.

1 School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey LL59 5AB, UK

Turbulent Dissipation Rates, Mixing, and Heat Fluxes in the

Canadian Arctic from Glider-based Microstructure Measurements

SCHEIFELE B.1, WATERMAN S.1, CARPENTER J.2

1 University of British Columbia, Canada 2 Helmholtz-Zentrum Geesthacht, Germany

Understanding mixing rates in the Arctic Ocean allows us to estimate vertical heat

fluxes through the water-column which have the potential to significantly impact heat budgets

as well as ocean-sea ice and ocean-atmosphere interactions. We present new observations

consisting of 340 quasi-vertical microstructure profiles of shear and temperature variance

alongside profiles of finescale temperature and salinity in the Amundsen Gulf region of the

Canadian Arctic. We use these to characterize the variability of turbulent mixing rates in both

space and time, and to begin identifying the dominant physical processes responsible for mixing

in this region. The measurements were collected over two weeks by an autonomous glider in

August 2015, and they represent one of the most dense microstructure sampling schemes in the

Arctic to date. Profiles encompass the most prominent features of the Arctic water column,

including the warm Atlantic water layer at depths below 250 m, the halocline between the

Pacific and Atlantic water layers, and the surface mixed layer which exhibits a strongly

stratified base. From the microstructure measurements, we calculate ε and χ, the dissipation

rates of turbulent kinetic energy and thermal variance. Dissipation rates very across four orders

of magnitude but are generally very low; consequently, mixing tends to be inhibited everywhere

by the strong stratification.

Oral preference

Comparisons between transect and fixed point measurements in an

oceanic turbulent flow: comparisons between intermittency

parameters

SCHMITT F.G.1, KOZIOL L.1, ARTIGAS L.F.2, LIZON F.3

1 CNRS, Laboratory of Oceanology and Geosciences, France 2 Univ. Littoral Côte d’Opale, Laboratory of Oceanology and Geosciences, France

3 Univ. Lille, Laboratory of Oceanology and Geosciences, France

Oceanic processes possess important fluctuations over large ranges of spatial and temporal

scales. These fluctuations are related with the turbulence of the ocean.Usually, in turbulence, one

considers fixed point Eulerian measurements, or Lagrangian measurements following an elements of

fluid. On the other hand, in oceanography, measurements are often done from a boat operating over a

transect, where the boat is moving in the medium at a fixed speed (relative to the flow). This was the

case of the first clear evidence of Kolmogorov spectrum from a field experiment, performed in a tidal

stream near Vancouver [Grant et al., 1962].

Here the aim of our study is to consider if such moving reference frame is modifying the

statistics of the measurements. For this we compare two type of measurements at high frequency:

fixed point measurements, and transect measurements, where the boat is moving at a fixed speed

relative to the flow. 1 Hz fluorometer measurements are considered in both cases. Measurements

have been done the same day, under similar conditions.

Power spectra of time series are considered, as well as local mean and variance

measurements along each transect. It is found that the spectral scaling slope of the measurement is

not modified, but the variance is very different, being much larger for the moving frame. The

intermittency is also considered, using cumulant scaling analysis in order to compare the statistics

between fixed point and moving frame.

REFERENCES

Grant H. L., Stewart R. W. and Moilliet A., 1962. Turbulence spectra from a tidal channel. Journal of Fluid

Mechanics, 12(2), 241-268.

No preference

Shallow stratified shelf sea turbulence and mixing rates measured

by autonomous underwater gliders

SCHULTZE L., MERCKELBACH L., CARPENTER J.

Helmholtz-Zenturm Geesthacht, Germany

The quantification of turbulence and mixing rates in the field is a challenging task, in

that data should be ideally collected in such reliable manner that surrounding uncertainties, such

as instrument vibrations, ship wakes and other factors that could potentially disturb the

measurements, can be disregarded. In this study, two data sets spanning 33 days collected by

autonomous underwater gliders are presented. The gliders were equipped with a microstructure

package to measure turbulence dissipation rates in the German Bight of the North Sea under

strongly and weakly stratified, and well-mixed conditions. Strong tidally-driven bottom

boundary layer turbulence was observed, which reached up until the thermocline and was

subsequently damped by sharp and relatively weak stratification. Squared buoyancy frequencies

as low as 10-5 s-2 were found to reduce the buoyancy Reynolds number from a fully

turbulent state (Reb > 100) to an intermediate turbulence level (7 < Reb < 100) [Shih

et al., 2005]. Within the thermocline, the dissipation rate of turbulent kinetic energy was

intermittent, however the flow was mainly marked by laminar to low turbulent conditions (Reb

< 7). Vertical mixing took place at a relatively constant background rate with discernible peaks

of stronger diapycnal fluxes, which appear to account for much of the change in temperature in

the bottom mixed layer. Moreover, the thermocline acts as a natural barrier between surface and

bottom mixed layers, controlling vertical fluxes and turbulence. A distinct sub-surface

chlorophyll maximum was observed in the bottom of the thermocline, which might have been

formed due to favourable light, gases and nutrient conditions, together with the relatively

quiescent environment encountered [Ross and Sharples, 2007].

REFERENCES

Ross, O. N., and J. Sharples (2007), Phytoplankton motility and the competition for nutrients in the

thermocline, Marine Ecology Progress Series, 347, 21–38, doi:10.3354/meps06999.

Shih, L. H., J. R. Koseff, G. N. Ivey, and J. H. Ferziger (2005), Parameterization of turbulent fluxes and scales

using homogeneous sheared stably stratified turbulence simulations, Journal of Fluid Mechanics, 525

(August 2004), 193–214, doi:10.1017/S0022112004002587.

No preference

Slope-induced tidal straining: Analysis of rotational effects

SCHULZ K.1, ENDOH T.2, UMLAUF L.3

UTokyo Ocean Alliance, The University of Tokyo, Tokyo, Japan

Leibniz Institute for Baltic Sea Research, Warnemuende, Germany

NIOZ Netherlands Institute for Sea Research, and Utrecht University, Yerseke

Estuaries and many regions of the coastal ocean are characterized by a persistent horizontal

density gradient, typically generated by river-runoff, differential heating, or horizontal variations in

turbulent mixing. The interaction of this externally imposed gradient with a tidal current leads to a periodic

stabilization and destabilization of near-bottom stratification – with important implications for near-bottom

turbulence. The resulting tidal asymmetries in turbulent mixing feed back on the momentum budget,

creating residual currents and transports of suspended sediment. In a recent model study (Schulz and

Umlauf, 2016), a similar process called “slope-induced” tidal straining has been investigated, which is

generated by the projection of the vertical density stratification onto a sloping bottom, and does not require

an externally imposed density gradient. When the tidal current is directed up the slope, dense water is

advected on top of lighter water by friction-induced shear, creating a gravitationally unstable and thus

strongly turbulent near-bottom layer. Vice-versa, when the current reverses, stable stratification is

established and turbulence is suppressed. Again, this induces a residual current and suspended matter

transport, which is in most cases directed up slope with transport rates of several kilometers per tidal cycle.

A recent data from the East China Sea (Endoh et. al. 2016) provides first observational evidence

for slope-induced tidal straining on a continental shelf. To reproduce these observations, the model above

has been extended to include Earth rotation effects, which had previously been neglected. In the present

study, it is shown that this simple, one-dimensional model can reproduce the key features of the

observations. The effect of changes in rotation rates and other key parameters on the effective transport of

suspended material are discussed.

Oral preference

1 UTokyo Ocean Alliance, The University of Tokyo, Tokyo, Japan

2 Leibniz Institute for Baltic Sea Research, Warnemuende, Germany

3 NIOZ Netherlands Institute for Sea Research, and Utrecht University, Yerseke, The Netherlands

Energy and mixing in stratified turbulent flows

A. Scotti and P.-Y. PassaggiaDept. of Marine Sciences, UNC, Chapel Hill, NC

Energy-based arguments provide a fundamental diagnostic tool in turbulent flows. In incompressibleflows where the density ρ is constant, the energy is the familiar kinetic energy Ek = ρu2/2, which enjoysthe property of being a convex function on the phase space. It can thus be readily split into a mean andturbulent (or eddy) component once an appropriate notion of average is introduced. The insight comes byan analysis of the sink and source terms in the appropriate transport equations for the mean and turbulentcomponents. In stratified (but still incompressible, i.e. Bussinesq) flows there is a second reservoir of energy,the potential energy of the flows. However, the standard expression Ep = ρgZ for the latter is not convex,and thus, within an energetic framework based on this definition, turbulent fluctuations have zero potentialenergy, a rather unsatisfactorily state of affairs. Further, there is no term in the equations that representsa truly irreversible sink of potential energy. The solution is to modify the definition of potential energyto include only the portion that is effectively available. This introduces the concept of Available PotentialEnergy (APE), initially formulated by Margules [1903] for isolated storms and later extended by Lorenz[1955] to the entire atmosphere. Lorenz APE assumes small deviations from equilibrium (hence, the APEis quadratic in density fluctuations). Subsequent developments removed the small-amplitude requirement[Andrews, 1981, Holliday and McIntyre, 1981], extended the APE framework to diagnose overall mixingin a closed system via a global APE [Winters et al., 1995], and localized mixing with a local APE [Scottiand White, 2014]. Along the way, Shepherd [1993] showed the connection between APE and the Casimirsof the Hamiltonian that describes the adiabatic dynamics. A common feature of all APEs proposed sofar is that the APE depends only on the buoyancy distribution, and not on the combined buoyancy andmomentum distribution. In this talk, we approach the problem of APE using a general framework based on avariational approach that uses the Casimir functionals of the Hamiltonian as Lagrange multipliers [Scotti andPassaggia, 2017]. It recovers the standard buoyancy-only dependent formulation in non-rotating systems.Unlike the standard formulation, rotation (and other) effects can be included in a natural way, in which casethe Available Energy depends both on the buoyancy and potential vorticity distribution. We discuss someapplications to a simple Eady problem Eady [1949]. We show that mixing is intimately connected to theevolution of the Casimir functionals under diabatic conditions.

References

D. G. Andrews. A note on potential energy density in a stratified compressible fluid. J. Fluid Mech., 107,1981.

E. Eady. Long waves and cyclone waves. Tellus, 1:33–52, 1949.D. Holliday and M. E. McIntyre. On potential energy density in an incompressible stratified fluid. J. FluidMech., 107:221–225, 1981.

E. N. Lorenz. Available potential energy and the maintenance of the general circulation. Tellus, 7:157–167,1955.

M. Margules. Uber die Energie der Sturme. Jahrb. k. k. Zent.-Anst. fur Meteorol. und Erdmagnet., 48:1–26,1903.

A. Scotti and P.-Y. Passaggia. On the role of lagrangian invariants in the energetics of fluids. J. Fluid Mech.,Submitted:Preprint available as arXiv preprint, 2017.

A. Scotti and B. White. Diagnosing mixing in stratified turbulent flows with a locally defined availablepotential energy. J. Fluid Mech., 740:114–135, 2014.

T. G. Shepherd. A unified theory of available potential-energy. Atmosphere-Ocean, 31:1–26, 1993.K. B. Winters, P. N. Lombard, J. J. Riley, and E. A. D’Asaro. Available potential energy and mixing in

density stratified fluids. J. Fluid Mech., 289:115–128, 1995.

1

On the Relationship Between Turbulent Cascades and Eddy Tilts

NATASHA SENIOR

University of East Anglia, UK

Owing to its similarity with the 2D vorticity equation, the quasi-geostrophic equation permits the

existence of a dual cascade of energy and enstrophy. This is characterised by the formation of large-scale

structures, a consequence of energy being transported to successively larger scales through local interactions

between eddies of similar wavenumber. The spontaneous formation of zonal jets in the oceans could be

explained as intermediate or final states in the evolution of an inverse energy cascade. Jets are also known to be

supported by an effective negative viscosity, where the divergence of the Reynolds' stress pattern reveals eddies

tilting in such a manner to flux momentum into the mean flow. What is unclear is how these eddy-mean-flow

interactions fit in to the picture of geostrophic turbulence. Using a channel model forced at small scales and

dissipated through Rayleigh friction, the relationship between eddy-tilts and the formation of turbulent zonal jets

is investigated.

Norwich Research Park, Norwich, NR4 7TJ

Poster preference

Regional dynamics influence on small-scale mixing in the boundary current regions of the

North Western Mediterranean and the northwestern Japan Sea

SHATRAVIN A.V., OSTROVSKII A.

P.P. Shirshov Institute of Oceanology, Russian Academy of Science

Here we study relationship between mesoscale and submesoscale dynamics and

temporal variability of small-scale mixing parameters in the ocean northern boundary current

region. The data were obtained during two deployments of the Aqualog moored profiler: in

Besos Canyon (the north-western Mediterranean Sea) in March 2012 and in the northern part of

the Sea of Japan in April-October 2015. The thermohaline and hydrodynamic conditions in two

regions have much in common featuring the overall cyclonic circulation, meanders of the strong

longshore currents, numerous eddies of various spatial scales and near-coast trapped waves.

The profiler was equipped with the Nortek Aquadopp Doppler current meter and the SBE CTD

52MP probe providing us with high-resolution (about 1 meter) vertical profiles of current

velocities, salinity and temperature several times a day. The data were used to estimate the

diapycnal turbulent eddy diffusivity of mass based on finescale parameterization approach

[Polzin, 1995]. For the data processing we applied the Mixing Oceanographic Toolbox code

[Meyer et al., 2014] modified to process the Aqualog data. The vertical distributions of

diffusivities varied in time during the observational surveys. We analyze relation between the

depth-time variability of diffusivities and the thermohaline stratification in the anticyclonic and

cyclonic eddies as well as the peculiarities of the ocean current dynamics. The comparative

analysis of the estimates obtained for two ocean regions appeared to be particularly useful for

better understanding of mixing sources and the role of regional dynamics in the mixing

distribution. The seasonal variability of the mixing parameters is also described for the Japan

Sea conditions.

REFERENCES

Meyer A., Phillips H.E., Sloyan B.M. and Polzin K.L., 2014. Mixing (MX) Oceanographic Toolbox for EM-

APEX float data applying shear-strain parameterization. Institute for Marine and Antarctic Studies Tech.

Rep., 69 pp.

Polzin K.L., Toole J.M. and Schmitt R.W., 1995. Finescale parameterizations of turbulent dissipation. Journal

of physical oceanography, 25(3), pp.306-328.

Poster preference

Marginal instability and deep cycle turbulence in the equatorial Pacific

cold tongue

SMYTH W.1, MOUM J.1, PHAM H.2, SARKAR S.2

1 Oregon State University, USA

2 University of California, San Diego, USA

The upper equatorial oceans provide a natural laboratory for the study of turbulence in stratified,

parallel shear flows. The combination of steady trade winds and vanishing Coriolis acceleration leads

to persistent, sheared currents, and thereby supports a turbulence regime that is roughly in equilibrium

with the forcing - the so-called "deep cycle" of equatorial turbulence. The downward heat flux carried

by this turbulence is a critical element of the equatorial climate system and is notoriously difficult to

represent in climate models. In this talk I’ll describe the discovery of the deep cycle in the 1980s and

efforts to explain its physics, including our current understanding and its implications for stratified

shear flows in general (e.g. estuarine flows, gravity currents).

Oral preference (keynote)

1 Oregon State University, USA

2 University of California, San Diego, USA

Langmuir cells and ramp-like structures in the upper ocean turbulent

boundary layer

SOLOVIEV A.V.1,2, DEAN C.W.1, LUKAS R.2, DONELAN M.A.3, TERRAY E.4

1 Nova Southeastern University, Halmos College of Natural Sciences and Oceanography, USA 2 Department of Oceanography University of Hawaii, USA

3 Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA 4 Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, USA

Langmuir circulation has been recognized as an important part of the upper ocean turbulent boundary

layer [McWilliams et al. 1997]. The traditional models of Langmuir circulation, however, do not account for

ramp-like structures [Soloviev and Lukas 2014]. The ramp-like structures are widespread features in the upper

ocean turbulent boundary layer, with rotational axes perpendicular to the axes of Langmuir circulation [Thorpe

1985: Soloviev 1990; Wijesekera et al. 1999]. Vortices associated with ramp-like structures have transverse axes,

while Langmuir circulations have a longitudinal axis, relative to the wind. How can ramp-like structures coexist

with Langmuir cells? We are reporting a new concept, which links ramp-like structures and wave-breaking

turbulence to Langmuir circulation and the Craik and Leibovich [1976] model. Using computational fluid

dynamics tools, we have been able to reproduce both Langmuir cells and ramp-like structures coexisting in space

though intermittent in time. The mechanism resembles convective initiation of longitudinal rolls in the

atmospheric boundary layer [Brown 1991]. However, in the upper ocean turbulent boundary layer, the vorticity

transport in the vertical direction is caused by breaking waves stirring the near-surface layer of the ocean; while,

in the atmospheric boundary layer, by penetrative convection. The proposed mechanism for Langmuir

circulation is expected to be most efficient under developing seas, including high wind speed conditions when

the Stokes vortex force term can be partially suppressed due to strong near-surface turbulence [Terray et al.

1999]. We also discuss the available observational data that can help with the verification of models of spatially-

coherent organized structures in the upper ocean turbulent boundary layer.

REFERENCES

Brown, R.A., 1991. Fluid Mechanics of the Atmosphere. International Geophysics Series 47, Academic Press, San Diego.

Craik, A.D.D., and Leibovich, S., 1976. A rational model for Langmuir circulations. J. Fluid Mech.. 73, 401–426.

McWilliams, J.C., Sullivan, P.P., and Moeng, C-H., 1997. Langmuir turbulence in the ocean. J. Fluid Mech., 334, 1-30.

Soloviev, A.V., 1990. Coherent structure at the ocean surface in the convectively unstable conditions. Nature 346, 157–160.

Soloviev A.V., Lukas, R., 2014. The Near-Surface Layer of the Ocean: Structure, Dynamics, and Applications (Second

edition), Springer, NY 552 pp.

Terray, E.A., Drennan W.M., Donelan M.A., 1999. The vertical structure of shear and dissipation in the ocean surface layer.

In: M.L. Banner (ed.) The Wind-Driven Air-Sea Interface, School of Mathematics, University of New South Wales,

Sydney, Australia, 1999, 239-245.

Thorpe, S.A., 1985. Small-scale processes in the upper ocean boundary layer. Nature, 318, 519-522.

Wijesekera, H.W., Paulson, C.A., and Huyer, A., 1999. The effect of rainfall on the surface layer during a westerly wind

burst in the western equatorial Pacific. J. Phys. Oceanogr., 29, 612–632.

Oral preference

1 Nova Southeastern University, Halmos College of Natural Sciences and Oceanography, 8000 North Ocean Dr.

Dania Beach, FL 33004, USA

2 University of Hawaii, Department of Oceanography, 1000 Pope Road, Honolulu, HI 96822, USA

3 Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway,

Miami, FL 33149, USA

4 Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole,

MA 02543, USA

Horizontal dispersion in shelf seas: high resolution modelling as an

aid to sparse sampling

STASHCHUK N.1, VLASENKO V.1, INALL M.E.2, ALEYNIK D.2

1 University of Plymouth, Plymouth, UK 2 SAMS, Oban, UK

The ability of a hydrodynamic model to reproduce the results of a dye release

experiment conducted in a wide shelf sea environment was investigated with the help of the

Massachusetts Institute of Technology general circulation model (MITgcm).

In the field experiment a fluorescent tracer, Rhodamine WT, was injected into the

seasonal pycnocline, and its evolution was tracked for two days using a towed undulating

vehicle equipped with a fluorometer and a CTD.

With a 50 m horizontal resolution grid, and with three different forcings initialised in

the model (viz: tides, stationary current, and wind stress on the free surface), it was possible to

replicate the dye patch evolution quite accurately. The mechanisms responsible for the

enhancement of horizontal dispersion were investigated on the basis of the model results.

It was found that enhancement of the dye dispersion was controlled by vertically

sheared currents that, in combination with vertical diapycnal mixing, led to a substantial

increase in the "effective'' horizontal mixing.

The values of "effective'' horizontal mixing found from the model runs were in good

agreement with those obtained from in-situ data, and the probable degree to which the

observational techniques under-sampled the dye patch was revealed.

Oral preference

Mathematical versus physical constraints on ocean mixing

parameterisations.

RÉMI TAILLEUX1,

1Dept of Meteorology, University of Reading, United Kingdom

The development of turbulent mixing parameterisations in numerical ocean models relies

on two distinct approaches: the first one is theoretical in nature and is primarily concerned with

rooting the parameterisations in sound physical principles; the second one is empirical in nature, and

seeks to exploit available observations to optimally constrain the various adjustable parameters and

constants that inevitably enter most mixing parameterisations. In the first part of this work, we

investigate which components of rotated diffusion tensors can be inferred --- at least in principle ---

from the knowledge of the full turbulent heat and salt fluxes, assuming that the latter can be

constrained from observations. The main result is that the answer depends on whether equal or

different diapycnal diffusivities are assumed for heat and salt. If identical diffusivities are assumed,

as is most commonly done, then the knowledge of the turbulent heat and salt fluxes (referred to as

‘observations’ in the following) is found to constrain both the anti-symmetric and symmetric parts of

rotated diffusion tensors. Regarding the former, which is related to the meso-scale eddy-induced

advection, observations are able to constrain both the direction of the velocity potential vector and its

amplitude. Regarding the latter, observations constrain not only the isopycnal and diapycnal mixing

coefficients, but also the unit vector controlling the isopycnal and diapycnal mixing directions. If

different diffusivities are assumed, which corresponds to so-called differential diffusion, then

observations are able to constrain the isopycnal as well as the two diapycnal diffusivities; in that case,

the values obtained depends on the choice of mixing directions, which can be chosen arbitrarily

either to align with isopycnal and diapycnal directions, or with the horizontal/vertical directions. In

the second part of this work, we will investigate the feasibility of using observations for constraining

the full turbulent heat and salt fluxes. Two important implications of this work will be discussed. The

first discusses some of the potential missing ingredients of current meso-scale eddy

parameterisations, and in particular of the widely used Gent-McWilliams parameterisations. The

second discusses the validity of the widely held belief that the mixing directions in rotated diffusion

tensors should be parallel and perpendicular to the local neutral vector.

1 Dept of Meteorology, University of Reading, United Kingdom

Turbulence controls size distribution of aggregates: in-situ

observations by a microstructure profiler and a cabled observatory

MARIKA TAKEUCHI1, MARK J DOUBELL2, GEORGE JACKSON3 AND HIDEKATSU YAMAZAKI1

1 Tokyo University of Marine Science and Technology, Japan 2 South Australia Research and Development Institute, Australia

3 Texas A & M University, USA

Marine aggregates are ubiquitous particles formed from the accretion of smaller biogenic

and non-biogenic components. Visible aggregates, known as marine snow, are typically in the 0.5 to

few mm size range. Aggregates formation is an important pathway for transferring materials from

surface to the deep ocean [Buesseler& Boyd, 2009]. Aggregates are well recognised as hotspot of

microbial and planktonic activities [Azam & Long, 2001]. Turbulence is an important physical

mechanism in the aggregates formation and destruction. However, the relative roles of turbulence in

aggregates formation and destruction have not been clarified due to few observational studies. In this

study, we analysed simultaneous in-situ observations of turbulence and aggregate in the ocean and in

a lake. A microstructure profiler, TurboMAP-L, was used to collect shear data and a digital still

logger camera was used to collect images of aggregates. Digital images were subsequently used to

determine particle abundances and size distributions. Direct comparison of turbulence intensity and

aggregate size distributions show that turbulence below 𝜀=10-6[W/kg] enhances aggregation,

increasing average particle size; greater turbulence causes particle breakup, limiting the average

maximum aggregate size and decreasing the slopes of size distributions. This indicates the role of

turbulence controlling aggregate size distributions. We also present observations from a cabled

observatory further substantiate these conclusions.

REFERENCES

Azam, F. and Long, R.A., 2001. Oceanography: Sea snow microcosms. Nature, 414,495-498

Buesseler, K.O., and Boyd, P. W., 2009, Shedding light on processes that control particle export and flux

attenuation in the twilight zone of the open ocean, Limnol. Ocenogr., 54(4), 1210-1232

1 Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan

108-8477 2 Aquatic Sciences, South Australian Research and Development Institute, West Beach, South Australia

5024, Australia 3 Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A.

Flow-limited diurnal vertical migration

TANAKA M.1, YAMAZAKI H.1, ENDO Y.2, GALLAGER S.3

1 Tokyo University of Marine Science and Technology / Japan 2 Tohoku University / Japan

3 Woods Hole Oceanographic Institution / USA

While zooplankton Diurnal Vertical Migration (DVM) is cued by light, zooplankton

behavioral responses to physical processes, such as flow pattern and turbulence, have been discussed

with limited observed data. How do the physical processes modify zooplankton DVM? We present

light-cued but flow-limited zooplankton DVM observed by a cabled observatory deployed on the

ocean bottom at 20 m depth near an isolated island in Japan, which simultaneously measured

biological and physical parameters. Zooplankton images taken by an in situ microscope camera,

Continuous Plankton Imaging and Classification System (CPICS), were used for classification and to

estimate population density. Mysids and ostracods appeared during only nighttime with maximum

density of 3,000 mysids (or ostracods) m-3, respectively. Optical backscatter signal from turbidity

sensor was intermittent and dramatically higher during nighttime than daytime. This suggests that the

zooplankton was the source of the high turbidity data. In fact, the increase in zooplankton abundance

at night was observed frequently over the observation period. However, this increase was

significantly reduced when wave height was over 1 meter (e.g. when typhoons or tropical cyclones

passed over) which roughly corresponds to a maximum wave orbital velocity of 0.3 m s-1 at the

observatory depth. In addition, even when background flow velocity with period longer than 60 s was

over 0.1 m s-1, the increase in zooplankton at night was significantly suppressed. We present that the

observed feature of zooplankton DVM associated with the fluid motion caused by waves and

currents.

Oral preference

Large-eddy simulations of the interaction between sub-mesoscale

eddies and three-dimensional turbulence

TAYLOR J.

University of Cambridge, United Kingdom

Sub-mesoscale eddies, with typical sizes ranging from 1-10 km, are ubiquitous and

important features in the upper ocean. Sub-mesoscales develop through a variety of fluid instabilities

and grow at the expense of the kinetic and potential energy associated with larger scale currents. At

the same time, sub-mesoscales exist in a very energetic environment where much smaller three-

dimensional turbulence is generated, in part, by surface winds, breaking waves and convection. Here,

I will describe recent work using large-eddy simulations to examine the interaction between sub-

mesoscale eddies and three-dimensional turbulence in the ocean mixed layer.

The simulations show that sub-mesoscales and three-dimensional turbulence interact in

two competing ways. First, sub-mesoscale currents drive the formation of very sharp fronts through

sub-mesoscale frontogenesis. These fronts develop very strong horizontal and vertical shear, which

eventually becomes unstable to shear instabilities and leads to intense subduction and three-

dimensional turbulence.

Sub-mesoscales can also act to suppress three-dimensional turbulence. As they extract

energy from the large-scale flow, sub-mesoscales can lead to a stable vertical density profile. This

stable stratification acts to suppress three-dimensional turbulence that would otherwise be generated

by surface wind and buoyancy forcing. The large-eddy simulations show that the re-stratifiation

process is very inhomogeneous, leading to patches of intense turbulence surrounded by relatively

quiescent waters.

The interaction between sub-mesoscales and three-dimensional turbulence has a strong

impact on biogeochemistry and the distribution of buoyant material. Recent work has indicated that

the competition between re-stratification and surface cooling can either trigger or delay

phytoplankton blooms [Taylor and Ferrari, 2011, Mahadevan et al., 2012, Taylor, 2016]. New

simulations reveal that buoyant tracers accumulate in regions of strong surface convergence and

subduction. These tracers preferentially sample regions with very intense three-dimensional

turbulence, and their resulting mixing rate is modified relative to neutral tracers. This has important

implications for the mixing and distribution of buoyant material such including micro-plastics, oil

droplets, and bubbles.

REFERENCES

Taylor, J.R. and Ferrari, R., 2011. Ocean fronts trigger high latitude phytoplankton blooms. Geophysical

Research Letters, 38(23).

Mahadevan, A., D’Asaro, E., Lee, C. and Perry, M.J., 2012. Eddy-driven stratification initiates North Atlantic

spring phytoplankton blooms. Science, 337(6090), pp.54-58.

Taylor, J.R., 2016. Turbulent mixing, restratification, and phytoplankton growth at a submesoscale eddy.

Geophysical Research Letters, 43(11), pp.5784-5792.

Oral preference

Wave-vortex interactions in rotating shallow water

THOMAS J., BUHLER O., SMITH S.

Courant Institute of Mathematical Sciences

In this talk I will present recent theoretical work examining various

possible interactions between fast inertia-gravity waves and slow balanced quasi-geostrophic

motions in a rotating shallow water. Using multi-scale asymptotic analysis, a set of evolution

equations will be presented for the potential vorticity. These equations capture the slow

dynamics of the rotating shallow water to a higher degree of accuracy than the lowest order

approximate model - the quasi-geostrophic equation. These new asymptotic reduced models,

whose validity is confirmed by numerical experiments, point out that fast waves can

energetically interact with slow balanced motions. The results from asymptotic models will be

complemented by a series of high resolution numerical simulations of the rotating shallow water

equations in regimes not directly accessible by asymptotic analysis, such as characterizing

turbulent wave-vortex interactions. The main findings of these simulations is that the presence

of strong waves can significantly impact the balanced motion. For instance, it is observed that

wave activity can prevent vortex mergers and inverse cascades, these being well known features

of balanced models such as the quasi-geostrophic equation.

Oral preference

AUV based study on physical and ecological processes at fronts

TIPPENHAUER S., WULFF T., VON APPEN W.J.

Alfred Wegener Institute

Small-scale processes and their effects get more and more attention when it comes to

understanding processes and changes in the (Arctic) ocean. Here we present a study on physical

processes and ecological responses at submesoscale frontal systems in the Fram Strait

investigated using an autonomous underwater vehicle (AUV). The AUV is equipped with

physical and biogeochemical sensors such as an acoustic Doppler current profiler, a turbulence

probe, a conductivity-temperature-depth probe, and sensors for Oxygen, Nitrate, Chlorophyll a,

and photosynthetically active radiation (PAR).

The study is designed such that the AUV covers tracks of several kilometers length in

cross-frontal direction with the front roughly located in the middle of the track. On its way, the

AUV records high-resolution vertical or zigzag profiles of the physical and biogeochemical

properties in the upper 50 m which includes the euphotic zone. In both, physical and

biogeochemical terms, the measurements revealed a complex structure of the water column. At

the fronts the distribution of phytoplankton and nutrients was highly inhomogeneous, possibly

due to wind-driven frontogenesis or the growth of mixed layer eddies. To set the observations

into a larger context we also examine ship-based and satellite data. We investigate how the

observed patterns of the potential vorticity and the biogeochemical properties may be formed

and which processes could lead to a smoothing of the observed gradients.

No preference

The role of barotropic to baroclinic tidal energy conversion: a view

towards improved turbulent mixing parameterisation in shelf Seas

TOBERMAN M.1, INALL M.1, POLTON J.2, PELLING H.2, PALMER M.2, RIPPETH T.3

1 The Scottish Association for Marine Science 2 National Oceanography Centre, Liverpool

3 University of Bangor

The seasonal pycnocline in temperate shelf seas represents a critical pathway for the

transport of heat, nutrient and carbon. Accurate parameterisation of pycnocline exchange processes is

therefore key to the accurate determination of biogeogeochemical fluxes in these systems. However,

it is well established that the turbulence closure schemes currently utilized in regional scale models

fail to reproduce, by orders of magnitude, the observed levels of turbulent dissipation within the

seasonal pycnocline of shelf seas.

We hypothesise that this deficiency in turbulent mixing may be addressed via the inclusion

of an additional function, representing the energy input via tidal energy conversion. We propose that

this parameterisation can be derived via a spatial relationship, between the forcing imparted on the

baroclinic flow from the barotropic tide, and statistical distributions of observed TKE dissipation

rates (ε).

Preliminary results obtained from the comparison of output from the AMM6O NEMO

model, and turbulence observations from vertical free falling probes deployed in the Malin Shelf

region, suggest the existence of such a relationship.

Our hope is that the development of this relationship into a practicable parameterisation

for turbulent mixing within regional scale models, will result in an improvement in the ability to

predict pycnocline behaviour in shelf seas.

Poster preference

Marine turbulence in nearshore and surfzone areas

TOORMAN E., OUDA M.

KU Leuven

Climate change and sea level rise create a lot of concerns with regard to the vulnerability of coasts all

around the world. However, few of the flood risk studies take into account the morphodynamic evolution of the

coast over the long-term period of the prediction. Indeed, the numerical simulation of beach morphodynamics is

a very challenging problem. It requires the calculation of forces by waves and currents on the shore. A neglected

aspect is the interaction between sediments and turbulence above the bottom. Even though it is has been

acknowledged for a long time that sediments in the water column cause drag modulation, it is not properly

accounted for in numerical models. At best, the energy loss to sediment transport is implicitly accounted for by

tuning the bottom friction parameter.

Based on a long time investigation of this problem, based primarily on laboratory flume test data and

the failure to simulate these experiments with any numerical model, including experience with the author’s

(finite elements) research code FENST-2D, the main bottleneck has been identified in the turbulence model,

which cannot properly take into account particle-turbulence interactions. Even in advanced models based on

two-phase flow theory where the k-epsilon model is adapted semi-empirically with extra dissipation terms, the

models fail because they neglect the fact that turbulence is no longer fully-developed in the bedload transport

layer and that the near-bottom boundary conditions for turbulent kinetic energy and its dissipation rate should be

adapted accordingly.

A new modelling strategy has been designed, applicable to large scale 3D coastal engineering studies,

consisting of a two-layer turbulence model (a new low-Reynolds mixing length model in the bottom layer and a

compatible new low-Reynolds k-epsilon model for the outer layer), including semi-empirical corrections for

particle-turbulence interactions. The new mixing-length model also lies at the basis of a new dynamic bottom

friction model, accounting for energy dissipation by the sediments above the bottom. The latter has already

successfully been implemented in a 2DH model for the Belgian coast and Scheldt estuary [Bi & Toorman, 2015].

Within the framework of the CREST - Climate Resilient Coast project (www.crestproject.be) attention

is given to the incorporation of wave energy dissipation by different mechanisms. One of the aims is to

implement the new bottom friction closure in a 3D wave-resolved modelling of beach morphodynamics. For this

purpose, the modelling strategy will be implemented in OpenFOAM. Eventually the model will be used to study

how beach erosion extracts energy from the waves and modifies their properties and their subsequent impact on

the coast during storm conditions.

REFERENCE:

Bi, Q. & E.A. Toorman (2015). Mixed-sediment transport modelling in the Scheldt estuary with a physics based bottom

friction law. Ocean Dynamics, 65:555-587.

Oral preference

Benthic turbulence in the deep waters of a large lake

CARY D. TROY1 AND DAVID CANNON1

1 Purdue University, USA

We report very near-bed mean flow and turbulence measurements from a deep water (55m

depth) site in Lake Michigan (USA), a 450km long by 135km wide lake. The measurements span a

full year, and the focus of this paper is on turbulence and mean flow properties within the bottom 1m

of the water column. These measurements were obtained from a burst-sampled 2MHz Nortek

Aquadopp HR profiler. For mean currents, observations show consistent logarithmic structure within

the bottom 1m, with modifications at low speeds for stratification and unsteadiness. A log-linear

formulation for mean current distributions is seen to improve roughness and drag coefficient

inferences, even when fitting distributions within the bottom 1m. The log-linear formulation is also

shown to explain diminished drag coefficients inferred from an outer, weakly-stratified layer. We

employ a four quadrant analysis involving acceleration and stratification length scales to quantify the

relative importance of flow acceleration/deceleration and stable/unstable stratification. This analysis

identifies stratification to be relatively unimportant for the well-mixed winter and spring periods, but

unsteadiness to be important for much of the deployment.

Estimates of turbulent kinetic energy, turbulent kinetic energy dissipation, and

autocorrelation length scales show that standard law-of-the-wall (LOW) distributions for these

quantities are approached for high current speeds (1 mab speeds of 10 cm s-1 at this site). Turbulent

kinetic energy dissipation estimates from -5/3 spectral fitting and structure function fitting are in

good agreement with one another but show departures from LOW estimates for low speeds.

As an application of the measurements, we perform simple 1-D mussel clearance modeling

to simulate the influence of invasive quagga mussels on passive water column constituents under the

turbulent conditions observed at the site. These simulations show that mussel filtration effects in

these deep, sluggish waters should be restricted to a compact near-bed layer corresponding to the

zone of active turbulence above the lake bed.

1 Lyles School of Civil Engineering, Purdue University, West Lafayette, IN (USA)

Boundary mixing in nontidal basins: Observations from the Baltic Sea

UMLAUF L., LAPPE C.

Leibniz-Institute for Baltic Sea Research

Diapycnal mixing in large nontidal basins is often assumed to be related to the effect of near-inertial

waves. While the role of near-inertial shear for the generation of shear instabilities in the stratified interior is

relatively well understood, much less is known about the impact of near-inertial motions on boundary mixing

processes in nontidal systems, mainly owing to the lack of appropriate observations. Here, an extensive data set

is discussed, describing the variability of boundary mixing induced by near-inertial motions near the sloping

topography of one of the main basins of the Baltic Sea. These data reveal the existence of a vigorously turbulent

bottom boundary layer of a few meters thickness covering the entire slope region above and below the

permanent halocline that constitutes the main obstacle for vertical transport in the Baltic Sea. Near-bottom

turbulence was driven by the near-inertial shear in the frictional boundary layer rather than by breaking of near-

inertial waves near critical slopes. Large regions of the bottom boundary layer remained strongly stratified, and

therefore, different from the traditional view of inefficient boundary mixing, mixing efficiencies reached values

typical for the ocean's interior.

Oral preference

Stimulated loss of balance and other mechanisms of wave–turbulence

interactions

VANNESTE J.

University of Edinburgh, UK

Internal waves, near-inertial waves in particular, are strongly affected by the turbulent mesoscale flow

in which they propagate. When advected and refracted, they in turn exert a feedback on the flow, resulting in

significant energy transfers between waves and flow. It has recently been proposed that these energy transfers

provide a sizeable sink for mesoscale kinetic energy and thus make a significant contribution to the ocean energy

budget comparable, for instance, to that of bottom drag. A key to this is that the waves are excited externally, by

winds and tides, and reach substantial amplitudes. This is contrast with mechanisms of spontaneous loss of

balance, in which waves are generated by the slow evolution of the flow in the absence of any high-frequency

forcing. While spontaneous generation has been shown to be very weak and most likely negligible except in

strongly ageostrophic flows, the new mechanisms in which the loss of balance is stimulated by a pre-existing

wave field are active in weak, nearly geostrophic flows.

In this talk I will discuss a simplified model of stimulated loss of balance that couples the Young–Ben

Jelloul model of near-inertial waves with a quasi-geostrophic model for the flow (joint work with Xie). The

model captures both the advection and refraction of the waves by the flow, and the wave feedback which

is described by a modification of the familiar relation between potential vorticity and velocity of quasi-

geostrophic theory. This form of wave feedback is consistent with the predictions of generalised-lagrangian-

mean (GLM) theory which provides a convenient route for the computation of the wave feedback. I will also

discuss an extension due to Wagner & Young that incorporates the nonlinear generation of harmonic waves with

frequency 2f.

When propagating in a turbulent flow, waves are scattered by advection and refraction, leading to a

statistically isotropic wave field with well defined characteristics. Understanding this process is important for the

quantification of wave-flow energy transfers as well as for the separation between flow and waves that is often

requiring to analyse observations of ocean fields, e.g., from satellite altimetry. Time permitting I will explain

how this scattering can be described by a relatively simple kinetic equation obtained by modelling the turbulent

flow as a homogeneous random process (joint work with Danioux and Savva).

Oral preference

Kinematics of non-spherical particles in turbulence: effect of size and shape

VARIANO E., PUJARA N., BORDOLOI A.

University of California, Berkeley

Natural particles suspended in water are often non-spherical. We explore the ways in

which particle size and shape affects particle motion, focusing on particle parameters relevant

for plankton, sediment aggregates, or autonomous vehicles. We find that shape has only a very

weak effect on particle angular velocity, which is a quantity calculated with respect the global

reference frame (i.e. east/north/up). If we analyze rotation in a particle's local frame (i.e. the

particle's principle axes of rotation), then particle shape has a strong effect on rotation. In the

local frame, rotation is described by two components: tumbling and spinning. We find that rod-

shaped particles spin more than they tumble, and we find that disc-shaped particles tumble more

than they spin. These preferential rotations, as well as total angular velocity, decrease with

increasing particle size. Such behavior is indicative of how particles respond to the directional

influence of vortex tubes in turbulence, and such response has implications for particle motion

other than rotation. Understanding particle alignment is relevant for predicting particle-particle

collision rates, particle-wall collision rates, and the shear-driven breakup of aggregates. We

discuss these briefly in the context of what can be concluded from the rotation data discussed

above.

Presentation: Oral

Prediction of turbulent diapycnal mixing in density stratified flows

VENAYAGAMOORTHY S.K.

Colorado State University, USA

Quantitative prediction of diapycnal mixing of density and momentum in density stratified flows

remains an ongoing challenge in marine turbulence. From a practical standpoint, a major goal is the inference of

turbulent heat and momentum fluxes using indirect measurements in field studies of oceanic flows. To this end,

two key quantities need to either be measured or inferred. These are: (i) the rate of dissipation of turbulent

kinetic energy ε, and (ii) the flux Richardson number Rf (also commonly referred to as the mixing efficiency),

which is a measure of the amount of turbulent kinetic energy that is irreversibly converted into background

potential energy. A common (indirect) approach to estimates ε in oceanic flows is to assume a linear relationship

between the Thorpe (vertical overturn) length scale LT and the Ozmidov scale LO. This approach is popular since

the vertical scales of overturns are easily calculable using a sorting algorithm from inversions in standard density

profiles obtained from Conductivity-Temperature-Depth (CTD) measurements in the ocean. Hence the Thorpe

scale is essentially a kinematic scale that provides a description of the turbulence at a given sampling location

and instant in time. On the other hand, The Ozmidov scale is obtained from dimensional reasoning based on the

assumption that there is a balance between inertial and buoyancy forces. In other words, the Ozmidov scale is a

representative dynamic length scale of the largest eddy that is unaffected by buoyancy. A review of a number of

recent studies that were conducted in our research group will be presented in this talk to highlight the lack of a

linear relationship between the Thorpe length scale and the Ozmidov scale. These studies indicate that inferred

estimates of the dissipation rate of turbulent kinetic energy may be biased high by up to an order of magnitude or

more especially for large overturns in the ocean. An alternative framework using a two-dimensional parameter

space based on a buoyancy strength parameter (i.e. an inverse Froude number) and a shear strength parameter

will be presented to characterize the scaling correspondence of the overturning scale with pertinent turbulent

length scales. The second key quantity that is a necessary ingredient for the inference of diapycnal mixing from

oceanic measurements is the flux Richardson number Rf. To date, however, no unifying parameterization of Rf

exists due to both the variability inherent in geophysical flows as well as certain ambiguities that are introduced

in descriptions based on ill-conditioned single parameters. A discussion on the mixing efficiency and

implications for estimates of diapycnal mixing in marine turbulence will be presented.

Oral preference

What is the role of mixing in controlling microphytoplankton

community composition?

VILLAMAÑA M.1, MOURIÑO-CARBALLIDO B.1, MARAÑÓN E.1, CERMEÑO P.2, CHOUCIÑO P.1, ESTRADA

M.2, FERNÁNDEZ-CASTRO B.1, G. FIGUEIRAS F.3, OTERO-FERRER J.-L.1, REGUERA B.4

1Universidade de Vigo, Spain 2Institut de Ciències del Marn, Spain

3Instituto de Investigacións Mariñas, Spain 4Centro Oceanográfico de Vigo, Spain

Turbulence and mixing largely determine the availability of light and nutrients in the water

column, which in turn control the composition of the phytoplankton community. Due to the difficulty

of quantifying turbulence in the field, previous attempts to investigate the relationship between

mixing, nutrient and light availability, and phytoplankton community composition have often relied

on indirect proxies for characterizing mixing conditions of the water column. Here we combine, for

the first time, a large dataset of dissipation rates of turbulent kinetic energy, measured by a

microstructure turbulence profiler, with data of inorganic nutrient concentration, and microscopy-

based phytoplankton taxonomy, in order to investigate the role of mixing in controlling

microphytoplankton community composition. Our dataset includes 76 stations visited in three

contrasting environments which covered a wide gradient of nitrate availability conditions: the

Galician upwelling region (NW Iberian Peninsula), the NW Mediterranean, and the tropical and

subtropical Atlantic, Pacific and Indian oceans. Vertical mixing, derived from microstructure

turbulence, was used to compute light and nutrient availability for phytoplankton cells in the upper

layer. Our results indicate that nutrient supply was more important than light availability in

controlling the composition of microphytoplankton. In agreement with the famous model proposed

by Margalef in 1978, the contribution of diatoms to biomass becomes larger with increasing nutrient

supply. Moreover, mixing regimes for diatoms and dinoflagellates coincide with the values suggested

in the conceptual model proposed by Margalef mandala. These results demonstrate that accurate

estimates of nutrient supply, which, particularly in oligotrophic surface waters, are better predictors

of nutrient availability than nutrient concentrations, are crucial to discern the role that environmental

factors play in the composition of phytoplankton.

Oral preference

1 Departamento de Ecoloxía e Bioloxía Animal, Universidade de Vigo, 36310, Vigo, Spain.

2 Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Passeig Maritim de la

Barceloneta, 37-49, E-08003, Barcelona, Spain.

3 Instituto de Investigacións Mariñas, Consejo Superior de Investigaciones Científicas, Eduardo

Cabello 6, 36208, Vigo, Spain.

4 Centro Oceanográfico de Vigo, Instituto Español de Oceanografía (IEO), Subida a Radio Faro 50,

36390, Vigo, Spain

Characterisation of mixing efficiency from microstructure

measurements in the Sicily Channel

VLADOIU A.1, BOURUET-AUBERTOT P.1, CUYPERS Y.1, FERRON B.2, SCHROEDER K.3, BORGHINI M.4,

BRYDEN H.5, BEN ISMAIL S.6

1L'OCEAN UPMC, Paris, France 2LPO, Ifremer , Plouzan, France

3ISMAR, Venice, Italy 4ISMAR, Lerici (SP), Italy

5Ocean and Earth Science, Southampton, United Kingdom 6Institut des Sciences et Technologies de la Mer, Tunis, Tunisia

Microstructure measurements taken with a Vertical Microstructure Profiler (VMP) at two

deep passages show that the Sicily Channel is a hotspot for turbulent mixing. Average dissipation

rates of turbulent kinetic energy are high due to shear instabilities, internal tides, baroclinic near-

inertial waves breaking and interactions with the rough topography. A contrast for the deeper layers

was found between the eastern passage, where there is strong mechanically driven turbulence with

high turbulence intensities, and the western passage, in regions of lower turbulent kinetic energy

dissipation rates and smaller turbulence intensities, where double diffusive processes are active.

This study aims to characterise mixing efficiency based on microstructure measurements,

in the context of these dynamical regimes. The variations in mixing efficiency are investigated in

order to characterise different mixing regimes (turbulence, double diffusive salt fingers, double

diffusive convection).

The dissipation rate of turbulent kinetic energy and the dissipation rate of thermal variance

were inferred from the microstructure shear and temperature sensors, respectively, and finescale

temperature and density gradients were obtained from the Seabird temperature and conductivity

sensors.

The resulting mixing efficiency compares qualitatively well with the Bouffard and

Boegman (2013) parameterisation. However, the mixing efficiency computation is sensitive to the

choice of parameters, and the lack of a detailed method makes such a comparison problematic; an

optimal comparison should be conducted with the same choice of computation intervals and

parameters. At the eastern location, where mechanically driven mixing dominates, mixing efficiency

decreases with increasing turbulence intensity, and ranges from 1 to 0.005. When double diffusion

occurs (most often at the western location), values are up to 2 orders of magnitude higher than the

conventional value of 0.2. At high turbulence intensity (>103-104), the mixing efficiency is up to 1-2

orders of magnitude higher than the values obtained using the parameterisation.

Turbulence and double diffusion may occur simultaneously, and their relative contribution

to mixing is estimated, by comparing vertical turbulent diffusive fluxes to double diffusive fluxes.

Diapycnal diffusivity was inferred from the observations. The density variation per unit time

resulting from diffusive vertical mixing confirms the occurrence of significant water mass

transformations, notably a lightening of intermediate waters, which are diffusively diluted.

REFERENCES

Bouffard, D., and Boegman, L., 2013. A diapycnal diffusivity model for stratified environmental flows.

Dynamics of Atmospheres and Oceans, 61, 14-34.

Poster preference

1 L'OCEAN UPMC, Paris, France

2 LPO, Ifremer , Plouzan, France

3 ISMAR, Venice, Italy

4 ISMAR, Lerici (SP), Italy

5 Ocean and Earth Science, Southampton, United Kingdom

6 Institut des Sciences et Technologies de la Mer, Tunis, Tunisia

North Atlantic water overflow through the Wyville Thomson

Ridge: Observational evidence and numerical modelling

VLASENKO V., STASHCHUK N., NIMMO-SMITH A., HOWELL K.

University of Plymouth, UK

An overflow of dense cold water from the Faroe Bank Channel (FBC) through the Wyville

Thomson Ridge (WTR) into Rockall Trough was recorded during 136-th cruise of the RRS "James

Cook" in June 2016. Overflow event happened in the eastern most section of the ridge where

overflows were never recorded before. The Remotely Operated Vehicle ISIS deployed for collection

of bottom samples at 500m depth recorded abrupt drop of temperature. Before the event at 02:08am

on 5-th of June bottom water was nearly homogeneous below 400m depth with its temperature of

about 8.73OC and salinity 35.26 in the bottom 200m layer. Over 20 min water temperature dropped

to 6.74OC and salinity to 35.16, with fastest change of 1.10OC just over 1 minute 50 seconds. The

possibility of such a radical change of bottom sea water characteristics due a spillage of North

Atlantic bottom waters intruded into the FBC through the WTR is discussed based on the

observational data and the results of numerical modelling conducted using the Massachusetts

Institute of Technology general circulation model. The conditions of the recently recorded overflow

event are compared against the results published in [Stashchuk et al., 2010; 2011] where the analysis

of mixing processes in a downslope propagating gravity current, specifically, explosive detrainment

regime, is investigated using a passive tracer transport equation.

REFERENCES

Stashchuk N., V. Vlasenko, and T.J. Sherwin T.J., 2011. Numerical investigation of deep water circulation in

the Faroes Channel. Deep-Sea Research I, 58(7), 787-799.

Stashchuk N., Vlasenko V., and Sherwin T.J., 2010. Insights into the structure of the Wyville Thomson Ridge

overflow current from a fine-scale numerical model. Deep-Sea Research I, 57(10), 1192-1206.

Oral preference

Abyssal Turbulent Mixing in the Samoan Passage

GUNNAR VOET1, MATTHEW H. ALFORD1, JAMES B. GIRTON2, GLENN S. CARTER3 AND JODY M. KLYMAK4

1Scripps Institution of Oceanography, University of California San Diego, USA 2Applied Physics Lab, University of Washington, USA

3University of Hawaii, USA 4University of Victoria, Canada

The Samoan Passage is a major choke point for the Global Overturning Circulation. About 6 Sv (1 Sv ≡ 1 × 106 m3 s-1), more than half of the abyssal northward limb of the Pacific Meridional Overturning Circulation, flows through the Samoan Passage [Rudnick, 1997]. As the bottom current is forced through the narrow channels and across the sills of the Samoan Passage it is strongly modified by turbulent mixing. In our recent NSF-funded Samoan Passage Abyssal Mixing Experiment we found turbulent mixing in the Samoan Passage 1,000 to 10,000 times elevated above oceanic background levels [Alford et al., 2013]. High levels of mixing were strongly tied to hydraulic jumps and breaking internal waves behind the major sills [Voet et al., 2015].

A rich observational dataset, comprised of highly resolved moored and shipboard hydrographic and velocity measurements, as well as more than 100 full ocean-depth microstructure profiles, allows us to discern the role of these physical processes in the mixing of the abyssal current with its surrounding water masses. Hydraulic jumps lead to strong local turbulence whereas part of the energy removed from the mean flow and transferred to lee waves can radiate upwards. Lee waves also re-distribute momentum in the vertical. The detailed observations allow for an examination of the three-dimensional nature of flow and turbulent mixing over rough topography.

Hydraulic aspects of the flow through Samoan Passage, including the complex bathymetry with complicated sill configurations, multiple connecting passages and time dependence are now the topic of a newly NSF-funded project. We will conclude the presentation with a brief outlook on the upcoming work.

REFERENCES

Alford, M.H., Girton, J.B., Voet, G., Carter, G.S., Mickett, J.B. and Klymak, J.M., 2013. Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage. J. Phys. Oceanogr., 40, 4668-4674.

Rudnick, D., 1997. Direct velocity measurements in the Samoan Passage. J. Geophys. Res., 102, 3293-3302. Voet, G., Girton, J.B., Alford, M.H., Carter, G.S., Klymak, J.M. and Mickett, J.B., 2015. Pathways, Volume

Transport and Mixing of Abyssal Water in the Samoan Passage. J. Phys. Oceanogr., 45, 562-588.

1 Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla CA 92093, USA 2 Applied Physics Laboratory, 1013 NE40th Street, Seattle WA 98105, USA 3 University of Hawaii, 1000 Pope Road, Honolulu HI 96822, USA 4 University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada

Contribution of high and low frequency internal waves to boundary

turbulence in a lake

WAIN D.

University of Bath

Stratification in lakes restricts vertical mixing and often controls the spatial variability

of nutrients and other substances, affecting the distribution of dissolved oxygen in the water

column, the availability of nutrients to phytoplankton, and transport of pollutants between the

hypolimnion and epilimnion. The interior of lakes is often quiescent and most of the mixing in a

lake occurs at the sloping boundaries, where wind-induced internal waves create turbulence

(which leads to mixing) through interactions with the lakebed. To predict the occurrence and

strength of turbulence in terms of meteorological forcing and stratification, we investigated the

dependence of internal wave type, and their contribution to turbulence on the slope, on the Lake

number, which compares the stabilizing tendency of stratification to the destabilizing tendency

of the wind.

Three thermistor chains and a meteorological station were deployed in West Okoboji

Lake (length ~ 9 km, max. depth ~ 40 m) for two weeks. A wavelet analysis was conducted to

determine time periods when different wave frequencies were excited, with particular focus on

the first vertical mode seiche, the critical frequency with respect to the stratification and slope,

and high frequency waves in the band of 1-10 times the buoyancy frequency. We measured the

velocities in the bottom boundary layer (BBL) with a high resolution acoustic current profiler (2

MHz Nortek HR Aquadopp) and then computed the turbulent dissipation rate using the

structure function method, which uses the spatial correlations of velocity along a beam to

estimate the dissipation. This generated a two week time series of turbulent dissipation rate in

the BBL which was then compared to the wavelet amplitudes.

During the deployment, a strong daily wind forced near constant internal wave

activity. The theoretical period of the first vertical mode seiche was ~17 hours, but the diurnal

wind forcing interfered with free oscillation of this mode. Although not an obvious natural

frequency of the lake, waves of the critical frequency (which had a period of ~11 hours) were

activated throughout the measurement period. High-frequency waves were observed in the

thermistor chain near the slope at the lowest Lake number wind events. The turbulence

observed on the boundary was highest during these events, implying that the low frequency

seiching was less important than higher frequency motions in driving turbulence on the slope.

Oral preference

How climate changed driven turbulent mixing impact the water

quality dynamics: A case study in Chesapeake Bay, USA

MENG XIA, LONG JIANG

University of Maryland Easter Shore

In the partially-mixed estuary of Chesapeake Bay, the mechanisms of turbulent mixing in

regulating the biogeochemical cycles were given examination by using a newly developed three-

dimensional unstructured-grid hydrodynamic-eutrophication model (FVOCM-ICM, Xia and Jiang,

2016). Turbulent mixing was examined at the along-channel and across-channel directions from a

10-year (2003-2012) simulation, and was found to be very sensitive to the physical forcing and

climatic variability. Thus, the physical forcing projected by the CMIP5 (Coupled Model Inter-

comparison Project Phase 5) in the coming decades, including wind pattern shifts, river discharge

fluctuations, sea level rise, sea temperature increase, and salinity change in the ambient shelf waters,

were utilized to drive the coupled hydrodynamic-biogeochemical model. The difference between

past 10-year and forthcoming-decade turbulent mixing in the estuary was compared to quantify the

magnitude and direction of potential future changes. Impacts of these variations in turbulent mixing

on oxygen ventilation and regenerative nutrient (nitrogen, phosphorus, and silicon) transport were

further discussed. The statistical relationship between mixing intensity and nutrient and oxygen

budgets were established, which provides reference to the stakeholders and policy makers on future

water quality management activities.

Presentation : No Preference

Oceanic turbulence and highly intermittent phytoplankton dynamics

YAMAZAKI H1, MANDAL S2, TAKEUCHI M1, HOMMA H1 AND TANAKA M1

1 Tokyo University of Marine Science and Technology, Tokyo, Japan

2 Public Health Foundation of India, India

Phytoplankton requires both light and nutrient, thus requires to stay in the upper ocean where

turbulence stirs water column. Turbulence mixes oceanic properties, such as salinity and temperature.

How does it mix phytoplankton? How do they distribute in space? How small scale do we need to

resolve the microscale distribution? We have developed two types of fluorescence probe, LED (2 cm

resolution) and laser (2 mm resolution). We have found that the LED data are significantly different from

the laser data that exhibit highly intermittent features. The local signals are considerably stronger than the

spatially average signal. We also mounted an optic system on the microstructure profiler TurboMAP-L to

identify the source of intermittent fluorescence signals and found the strong signals came from marine

aggregates. From ten different field campaigns we compiled the average size of aggregate and the average

rate of kinetic energy dissipation rate and found they are positively correlated. The implications of the

field data will be presented in this talk. Also I will introduce new plankton ecosystem models that take

microscale spatial heterogeneity into account.

Oral preference

1 Tokyo University of Marine Science and Technology, Tokyo, Japan

2 Public Health Foundation of India, India

Surface horizontal diffusivity estimated from submesoscale

observations of surface currents and passive tracers

YOO J.G., KIM S.Y.

Korea Advanced Institute of Science and Technology

The optimal solutions)of surface horizontal diffusivity are quantified using submesoscale

[O(1) km spatial scale and O(1) hour time scale] observations of high-frequency derived surface

current fields and geostationary ocean color imagery-derived surface chlorophyll concentrations in a

coastal region, which correspond to the advection and concentration terms, respectively, in the one-

and two-dimensional advection-diffusion equations. We practice the ways to estimate the diffusion

coefficients using an idealized flow model and the model-derived concentrations. The model currents

are simulated uni-directional steady currents under specific temporal and spatial decorrelation scales

with a Gaussian noise, and the concentrations are derived using random walk and flight schemes,

which have been used for tracking of the concentration of phytoplankton, zooplankton, and

individuals of fishes as a statistical approach. Then, the diffusion equations are solved in terms of

random parameters and diffusion coefficients. This work will be applicable to the ecosystem process

studies at submesoscale and improve the limitation of the present-day statistical modeling.

Poster preference

Instabilities of vortices in thermal rotating shallow water model, and their

nonlinear saturation

ZEITLIN V.1, GOUZIEN E.2, LAHAYE N.2, DUBOS T.3

1 Laboratory of Dynamical Meteorology, Ecole Normale Superieure, Paris, France

2 Ecole Normale Superieure, Paris, France

3 Laboratory of Dynamical Meteorology, Ecole Polytechnique, Paris

We find a class of instabilities of convective character in the so-called thermal rotating shallow

water model, which is used in literature for modelling well-mixed boundary layers in the ocean and in the

atmosphere. These instabilities co-exist with standard barotropic instability, having higher than this latter

growth rates in a wide range of parameters. A whole variety of unstable modes is found with the help of

pseudo-spectral collocation method. The azimuthal structure of these modes strongly resembles that of the

centrifugal instability. Nonlinear evolution of the new instability was studied with th the help of numerical

simulations with the finite-difference scheme for rotating shallow water, which was appropriately modified.

We show that at nonlinear stage the instability produces characteristic pattern of convective instabilities,

which leads to rapid and efficient mixing inside the vortex. Dependence of the growth rates of these

unstable modes on the parameters: Rossby and Burger numbers, as well as on relative strength of pressure

and buoyancy gradients and their mutual orientation in the vortex is investigated.

Oral preference

Symmetric Instability (SI)-Turbulence: A Unique Form of Boundary Layer

Turbulence

THOMAS L.

Stanford University

Turbulence in the surface boundary layer is typically assumed to derive its kinetic energy

(KE) from the winds, buoyancy fluxes, and/or waves at the air-sea interface. This assumption can

break down at ocean fronts, where strong geostrophic shear associated with lateral density gradients

can supply KE to boundary layer turbulence via symmetric instability (SI). This SI-turbulence is thus

distinct from other forms of boundary layer turbulence in that it represents a sink of KE for the

balanced circulation. In this presentation, I will describe the instability criteria, energetics, coherent

structures, and turbulent fluxes associated with SI-turbulence and outline a model for its

parameterization. Observational examples of symmetrically unstable currents will be highlighted and

the implications for the dissipation of the KE of the circulation will be discussed. I will conclude by

speculating on the potential for SI-turbulence in the bottom boundary layer, where the intersection of

isopycnals with sloping bathymetry and friction can lead to favorable conditions for SI.

Keynote

A preliminary study of small scale turbulence and its association

with (sub-) mesoscale processes in the Denmark Strait overflow

plume.

Stylianos Kritsotalakis (1,2), Torsten Kanzow (1,2)

(1) Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven, Germany

(2) Bremen University, Department of Physics/Electrical Engineering, Bremen, Germany

The Denmark Strait overflow plume is a key component of the climate system in the North

Atlantic. During its descend into the subpolar North Atlantic, the plume almost doubles its

volume due to the entrainment of ambient waters. To study small-scale processes

associated with entrainment a multi-platform experiment was carried out 180 km

downstream of Denmark Strait in the pathway of the overflow plume. Moored observations

revealed pronounced eddy-activity with periods near 1.6 days (Schaffer et al, 2016). Using

velocity measurements from a high-frequency (1200KHz) ADCP we link the passage of

(sub-) mesoscale features within the plume with: a) the evolution of the internal wave field

and b) enhanced small scale turbulence events.

Fission of internal solitary waves over shoaling topography cascades tidal energy to turbulence

Zhiyu Liu (zyliu@xmu.edu.cn)

State Key Laboratory of Marine Environmental Science, and Department of Physical Oceanography, Xiamen University, Xiamen 361102, China

The tides are a major energy source of small-scale turbulence, and therefore diapycnal mixing, in the world's oceans. An understanding of the processes responsible for the cascade of energy from tides to turbulence is important in identifying when and where this mixing will take place. Internal solitary waves (ISWs) generated by tide-topography interactions are ubiquitous in the world's oceans and are thought to be important sources of mixing. Whilst the understanding of the dynamics and energetics of ISWs have been greatly advanced in the past a few decades, identification of the processes and mechanisms responsible for their dissipation is limited. Here we present velocity and turbulence measurements from the South China Sea, together with process-orientated numerical simulations, to demonstrate the key role of ISW fission, into groups of high-frequency internal waves over rough topography, in the dissipation of tidal energy. The results show that, as a result of the fission, wave-induced velocity shear is elevated over significant time periods coincident with a period of enhanced turbulent dissipation. We suggest that the enhanced dissipation is a result of instability and breaking of the high-frequency internal waves. The finding reveals an important pathway of tides-to-turbulence cascade and generation of turbulence and mixing in the ocean interior, having important implications for understanding ocean dynamics as well as its ecological and climatic impacts.

Regimes of oceanic turbulence in the Western Mediterranean represented

by satellite data

KARIMOVA S.

University of Liège, Belgium

Satellite observations provide encouraging opportunities for observing oceanic turbulence at a wide

range of spatio-temporal scales and through using different techniques and sensors. However, most studies in

this field are typically devoted to investigating multiscaling [e.g. Renosh et al., 2015] or multifractal [e.g. Pont et

al., 2013] properties of satellite, mostly thermal infrared, images. In the present work, we apply satellite data

both for observing turbulent features of surface currents and for an analysis of the mechanisms inducing such

features.

The main test site of the present study is the Western Mediterranean (WMed) Basin, which is known

for its well-developed turbulence giving rise to a cascade of coherent vortical structures. Turbulence in surface

waters of this area exhibits significant temporal variability, which provides additional stimulus to studying the

triggering mechanisms of the changes of the turbulent regime in the area under investigation. Observation of

turbulent features in the work being presented was performed via a registration of coherent vortical structures

manifested in two types of satellite imagery: (i) thermal infrared (namely, the L3S product of sea surface

temperature) and (ii) synthetic aperture radar (SAR) images, namely the Envisat Advanced SAR (ASAR)

imagery. Using such a dataset allows for observing coherent structures in the range O(1-100 km). The

cumulative temporal coverage of the region of interest by the imagery analysed is from 2008 to 2015. An

inspection of the images mentioned revealed in WMed an asymmetry of turbulent stirring typical for inner and

marginal seas [Karimova et al., 2017]. Thus, vortical structures with diameters less than approx. 70 km in

diameter mostly had a cyclonic sign of rotation, while those bigger than 70 km were mostly anticyclonic.

Studying the factors that trigger establishing different turbulent regimes in the region of interest was performed

via an analysis of concurrent information on spatial distribution of some dynamical and geophysical parameters

obtained from satellite observations and from the reanalysis. Additional satellite data scrutinized at this stage

among others include geostrophic and Ekman surface currents provided by different altimetry-derived products.

One of the most important parameters provided by the numerical simulations and analysed in the present work is

the mixed layer depth. Thus, it was discovered that most submesoscale eddies in the region of interest were

observed in quite shallow upper mixed layer, which can question mixed layer instabilities being a possible

mechanism of generation of such eddies.

This research is supported by the University of Liege and the EU in the context of the FP7-PEOPLE-

COFUND-BeIPD project. SAR imagery was provided by the European Space Agency. The study has been

conducted using E.U. Copernicus Marine Service Information.

REFERENCES

Karimova, S., 2017. Observations of asymmetric turbulent stirring in inner and marginal seas using satellite imagery. Int. J.

Remote Sens., 38(6), 1642-1664.

Pont, O., Turiel, A., and Yahia, H., 2013. Singularity analysis of digital signals through the evaluation of their unpredictable

point manifold. Int. J. Computer Mathematics, 2013, 90(8), 1693-1707.

Renosh, P.R., Schmitt, F.G., and Loisel, H., 2015. Scaling analysis of ocean surface turbulent heterogeneities from satellite

remote sensing: Use of 2D structure functions. PLoS ONE, 10(5): e0126975.

Poster preference

Oceanic Turbulence from a Planktonic Perspective

FRANKS P.

Scripps Institution of Oceanography, USA

Turbulence is often invoked as an essential dynamic controlling the interactions among the plankton.

Predator-prey and mating encounters are thought to be strongly influenced by the relative motions driven by

turbulence. However, analyses of a synthesis of more than 1.6 million turbulence dissipation measurements from

all over the world’s ocean show that in most of the ocean, most of the time, the turbulence is extremely weak,

with eddy viscosities close to molecular viscosity values. From the point of view of the plankton, the ocean is a

relatively tranquil environment – a view seemingly antithetical to most plankton ecologist’s intuition. The

strongly skewed statistics of dissipation rates suggest that only a small portion of a population experiences the

high velocities, shears, strains and vorticities associated with mean dissipation rates. Furthermore, plankton drift

with the flow, experiencing turbulence from a Lagrangian point of view. However, most of the theory exploring

the effects of turbulence on plankton has been developed from an Eulerian perspective, based on the relative

velocities at two points fixed in space. I will show analyses of a recent compilation of turbulence measurements

and attempt to introduce contemporary understanding of the statistics of turbulence and the Lagrangian

properties of turbulence from a planktonic viewpoint. My goal is to help plankton ecologists understand the

structure of turbulence and how it affects planktonic dynamics at different spatial scales. In particular I would

like to help biologists to move beyond the simple Kolmogorov scalings that are typically used, to obtain a more

nuanced understanding of the microscale structure of turbulence intensity. A better understanding of how

plankton experience the Lagrangian properties of turbulence will allow plankton ecologists to gain insights into

the evolution of planktonic behaviors and morphologies.

Keynote

The Annual Cycle of Upper-Ocean Potential Vorticity and its

Relationship with Submesoscale Instabilities: Insights from

Mooring Observations

YU X.1, NAVEIRA GARABATO A.1, MARTIN A.2, BUCKINGHAM C.3, BRANNIGAN L.4

1 University of Southampton, UK 2 National Oceangraphy Center Southampton, UK

3 British Antarctic Survey, UK 4 Stockholm University, Sweden

Potential vorticity (PV) is a key ingredient to understand the competing processes in deepening

and shoaling the ocean surface boundary layer (OSBL). Previous PV studies were mainly based

on numerical simulations. Here we use a year-long, meso- and submesoscale resolving time

series of buoyancy and horizontal velocity, obtained from mooring observations in the

Northeast Atlantic under the auspices of the U.K. OSMOSIS programme, to investigate the

upper-ocean PV budget and its relationship with submesoscale frontal instabilities. Results

show that non-advective PV changes in the OSBL are balanced by the frictional and diabatic

components of J vector in an integral sense. Deep mixed layers (up to about 350 m) during

wintertime are attributed to persistent atmospheric cooling, predominantly through gravitational

instability. However, on shorter time scales, conditions favourable to symmetric instability are

often observed when winds are aligned with the frontal flow. The ensuing overturning

instabilities rapidly re-stratify the mixed layer and limit the reduction of PV, as indicated by the

approximate balance between the temporal change of PV and the advection of PV below the

convective layer. These results emphasize the key role of submesoscale instabilities in

determining the evolution of the OSBL.

Oral preference

Turbulent dissipation at the western boundary of the Atlantic in an

eddy

FRAJKA-WILLIAMS, E. 1, EVANS, D. G. 1, CLEMENT, L. 2 AND NAVEIRA GARABATO, A. 1

1University of Southampton, United Kingdom 2Lamont-Doherty Earth Observatory, United States

Oceanic mesoscale eddies disappear from altimetry data at the western boundary of ocean

basins. While disappearance could mean a transfer of energy to other wave modes or mean flow in

the region, we investigate whether local dissipation is a leading order term in the mesoscale eddy

dissipation. From a short survey east of the Bahamas using a microstructure profiler, we identified

regions of elevated dissipation (10^-8, and occasionally exceeding 10^-7 W/kg) in the lee of

topography. These observations are consistent with estimates of dissipation derived using a fine

scale parameterisation from a moored 75 kHz ADCP in the RAPID 26N mooring array (Clement et

al., 2016). The 18 month record further showed that levels of estimated dissipation are higher in

anticyclones than cyclones. In addition to upward propagating internal waves, the survey also

identified 100 m-thick layers of high dissipation in a region of strong shear about 100 m off the

bottom. From the mooring and in situ observations, it is unclear whether a single candidate

mechanism dominates the local dissipation in an anticyclonic eddy. The level of mixing observed is,

however, O(100) times above background mixing levels in the ocean, and may represent an

important term in the mesoscale eddy energy budget.

REFERENCES

Clement, L., Frajka-Williams, E., Sheen, K. L., Brearley, J. A., Naveira Garabato, A. C., 2016. Generation of

internal waves by eddies impinging on the western boundary of the North Atlantic.. J. Phys. Oceanog., 46,

1067-1078.

1 Complete adress for the first institution using the « Adresses » style. 2 Second adress using the « Adresses » style.

3D hydrodynamical modelling of the Southern Bight of the North Sea: first

achievements and perspectives.

IVANOV E.1, CAPET A.1, BARTH A.2, DELHEZ E.3, SOETAERT K.4, GRÉGOIRE M.1

1 MAST, University of Liège, Belgium 2 GHER, University of Liège, Belgium 3 MMM, University of Liège, Belgium

4 Marine biology research group, UGent, Belgium

The impact of offshore wind farm installation and dredging activities on the spatial distribution and

dynamics of sediment grain size, biogeochemistry and biodiversity will be estimated in the Southern Bight of the

North Sea (SBNS) with a focus on the Belgian Coastal Zone (BCZ) in the frame of the FaCE-It research project

(Functional biodiversity in a Changing sedimentary Environment: Implications for biogeochemistry and food

webs in a managerial setting).

The three-dimensional hydrodynamical model ROMS-COAWST was implemented for simulation of

the complex hydrodynamics of SBNS and sediment transport. The first level of nesting with the resolution of 1

km was used in the area of Belgian Economical Zone. Six-hourly ECMWF ERA-interim meteorological data

was used to force the model at the sea-air boundary and the coarse resolution model results available from

Copernicus Marine Environment Monitoring Service were used to force the model at the open boundaries. Tides

and rivers were also considered.

Next types of long-run simulations have been conducted: a 10-years climatological simulation and an

interannual simulation over 2004-2013 in order to investigate the interannual dynamics. The model accuracy was

evaluated through validation of its outputs against observed salinity, temperature and currents data (remote

sensing and in-situ).

Results validation of currents and temperature and salinity horizontal fields and vertical profiles

against available satellite fields and in-situ data, i.e. from the project field campaign, is conducted and discussed.

Application of the nested grid and its benefits for results accuracy is also presented.